diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 6db48054..d90a63ad 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -9,7 +9,7 @@ repos:
   rev: 5.12.0
   hooks:
   - id: isort
-    
+
 - repo: https://github.com/pycqa/flake8
   rev: 6.1.0
   hooks:
diff --git a/rubicon_ml/client/artifact.py b/rubicon_ml/client/artifact.py
index 6b2cb44b..5a24bb81 100644
--- a/rubicon_ml/client/artifact.py
+++ b/rubicon_ml/client/artifact.py
@@ -1,6 +1,9 @@
+from __future__ import annotations
+
 import os
 import pickle
 import warnings
+from typing import TYPE_CHECKING, Optional
 
 import fsspec
 
@@ -9,6 +12,10 @@
 from rubicon_ml.client.utils.exception_handling import failsafe
 from rubicon_ml.exceptions import RubiconException
 
+if TYPE_CHECKING:
+    from rubicon_ml.client import Project
+    from rubicon_ml.domain import Artifact as ArtifactDomain
+
 
 class Artifact(Base, TagMixin):
     """A client artifact.
@@ -32,7 +39,7 @@ class Artifact(Base, TagMixin):
         logged to.
     """
 
-    def __init__(self, domain, parent):
+    def __init__(self, domain: ArtifactDomain, parent: Project):
         super().__init__(domain, parent._config)
 
         self._data = None
@@ -42,8 +49,8 @@ def _get_data(self):
         """Loads the data associated with this artifact."""
         project_name, experiment_id = self.parent._get_identifiers()
         return_err = None
-        for repo in self.repositories:
-            self._data = None
+        self._data = None
+        for repo in self.repositories or []:
             try:
                 self._data = repo.get_artifact_data(
                     project_name, self.id, experiment_id=experiment_id
@@ -56,7 +63,7 @@ def _get_data(self):
             raise RubiconException("all configured storage backends failed") from return_err
 
     @failsafe
-    def get_data(self, unpickle=False):
+    def get_data(self, unpickle: bool = False):
         """Loads the data associated with this artifact and
         unpickles if needed.
 
@@ -68,7 +75,7 @@ def get_data(self, unpickle=False):
         """
         project_name, experiment_id = self.parent._get_identifiers()
         return_err = None
-        for repo in self.repositories:
+        for repo in self.repositories or []:
             try:
                 data = repo.get_artifact_data(project_name, self.id, experiment_id=experiment_id)
             except Exception as err:
@@ -80,7 +87,7 @@ def get_data(self, unpickle=False):
         raise RubiconException("all configured storage backends failed") from return_err
 
     @failsafe
-    def download(self, location=None, name=None):
+    def download(self, location: Optional[str] = None, name: Optional[str] = None):
         """Download this artifact's data.
 
         Parameters
@@ -104,17 +111,17 @@ def download(self, location=None, name=None):
             f.write(self.data)
 
     @property
-    def id(self):
+    def id(self) -> str:
         """Get the artifact's id."""
         return self._domain.id
 
     @property
-    def name(self):
+    def name(self) -> str:
         """Get the artifact's name."""
         return self._domain.name
 
     @property
-    def description(self):
+    def description(self) -> str:
         """Get the artifact's description."""
         return self._domain.description
 
diff --git a/rubicon_ml/client/base.py b/rubicon_ml/client/base.py
index 736f0851..30be1b76 100644
--- a/rubicon_ml/client/base.py
+++ b/rubicon_ml/client/base.py
@@ -1,3 +1,13 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, List, Optional
+
+if TYPE_CHECKING:
+    from rubicon_ml.client import Config
+    from rubicon_ml.domain import DOMAIN_TYPES
+    from rubicon_ml.repository import BaseRepository
+
+
 class Base:
     """The base object for all top-level client objects.
 
@@ -9,19 +19,22 @@ class Base:
         The config, which injects the repository to use.
     """
 
-    def __init__(self, domain, config=None):
+    def __init__(self, domain: DOMAIN_TYPES, config: Optional[Config] = None):
         self._config = config
         self._domain = domain
 
-    def __str__(self):
+    def __str__(self) -> str:
         return self._domain.__str__()
 
     @property
-    def repository(self):
-        return self._config.repository
+    def repository(self) -> Optional[BaseRepository]:
+        return self._config.repository if self._config is not None else None
 
     @property
-    def repositories(self):
+    def repositories(self) -> Optional[List[BaseRepository]]:
+        if self._config is None:
+            return None
+
         if hasattr(self._config, "repositories"):
             return self._config.repositories
         else:
diff --git a/rubicon_ml/client/config.py b/rubicon_ml/client/config.py
index a7b41bcf..623e89df 100644
--- a/rubicon_ml/client/config.py
+++ b/rubicon_ml/client/config.py
@@ -1,8 +1,14 @@
 import os
 import subprocess
+from typing import Dict, Optional, Tuple, Type
 
 from rubicon_ml.exceptions import RubiconException
-from rubicon_ml.repository import LocalRepository, MemoryRepository, S3Repository
+from rubicon_ml.repository import (
+    BaseRepository,
+    LocalRepository,
+    MemoryRepository,
+    S3Repository,
+)
 
 
 class Config:
@@ -29,18 +35,22 @@ class Config:
     """
 
     PERSISTENCE_TYPES = ["filesystem", "memory"]
-    REPOSITORIES = {
+    REPOSITORIES: Dict[str, Type[BaseRepository]] = {
         "memory-memory": MemoryRepository,
         "filesystem-local": LocalRepository,
         "filesystem-s3": S3Repository,
     }
 
     def __init__(
-        self, persistence=None, root_dir=None, is_auto_git_enabled=False, **storage_options
+        self,
+        persistence: Optional[str] = None,
+        root_dir: Optional[str] = None,
+        is_auto_git_enabled: bool = False,
+        **storage_options,
     ):
         self.storage_options = storage_options
         if storage_options is not None and "composite_config" in storage_options:
-            composite_config = storage_options.get("composite_config")
+            composite_config = storage_options.get("composite_config", [])
             repositories = []
             for config in composite_config:
                 self.persistence, self.root_dir, self.is_auto_git_enabled = self._load_config(
@@ -62,7 +72,9 @@ def _check_is_in_git_repo(self):
                 "Not a `git` repo: Falied to locate the '.git' directory in this or any parent directories."
             )
 
-    def _load_config(self, persistence, root_dir, is_auto_git_enabled):
+    def _load_config(
+        self, persistence: Optional[str], root_dir: Optional[str], is_auto_git_enabled: bool
+    ) -> Tuple[str, Optional[str], bool]:
         """Get the configuration values."""
         persistence = os.environ.get("PERSISTENCE", persistence)
         if persistence not in self.PERSISTENCE_TYPES:
@@ -77,7 +89,7 @@ def _load_config(self, persistence, root_dir, is_auto_git_enabled):
 
         return (persistence, root_dir, is_auto_git_enabled)
 
-    def _get_protocol(self):
+    def _get_protocol(self) -> str:
         """Get the file protocol of the configured root directory."""
         if self.persistence == "memory":
             return "memory"
@@ -89,7 +101,7 @@ def _get_protocol(self):
 
         return "custom"  # catch-all for external backends
 
-    def _get_repository(self):
+    def _get_repository(self) -> BaseRepository:
         """Get the repository for the configured persistence type."""
         protocol = self._get_protocol()
 
diff --git a/rubicon_ml/client/dataframe.py b/rubicon_ml/client/dataframe.py
index d1c8394f..facef63f 100644
--- a/rubicon_ml/client/dataframe.py
+++ b/rubicon_ml/client/dataframe.py
@@ -1,7 +1,15 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Callable, Literal, Optional, Union
+
 from rubicon_ml.client import Base, TagMixin
 from rubicon_ml.client.utils.exception_handling import failsafe
 from rubicon_ml.exceptions import RubiconException
 
+if TYPE_CHECKING:
+    from rubicon_ml.client import Experiment, Project
+    from rubicon_ml.domain import Dataframe as DataframeDomain
+
 
 class Dataframe(Base, TagMixin):
     """A client dataframe.
@@ -24,14 +32,16 @@ class Dataframe(Base, TagMixin):
         logged to.
     """
 
-    def __init__(self, domain, parent):
+    def __init__(self, domain: DataframeDomain, parent: Union[Experiment, Project]):
         super().__init__(domain, parent._config)
 
+        self._domain: DataframeDomain
+
         self._data = None
         self._parent = parent
 
     @failsafe
-    def get_data(self, df_type="pandas"):
+    def get_data(self, df_type: Literal["pandas", "dask"] = "pandas"):
         """Loads the data associated with this Dataframe
         into a `pandas` or `dask` dataframe.
 
@@ -59,7 +69,12 @@ def get_data(self, df_type="pandas"):
         raise RubiconException(return_err)
 
     @failsafe
-    def plot(self, df_type="pandas", plotting_func=None, **kwargs):
+    def plot(
+        self,
+        df_type: Literal["pandas", "dask"] = "pandas",
+        plotting_func: Optional[Callable] = None,
+        **kwargs,
+    ):
         """Render the dataframe using `plotly.express`.
 
         Parameters
diff --git a/rubicon_ml/client/experiment.py b/rubicon_ml/client/experiment.py
index b71307d4..50803f07 100644
--- a/rubicon_ml/client/experiment.py
+++ b/rubicon_ml/client/experiment.py
@@ -1,3 +1,7 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
 from rubicon_ml import domain
 from rubicon_ml.client import (
     ArtifactMixin,
@@ -12,6 +16,10 @@
 from rubicon_ml.client.utils.tags import filter_children
 from rubicon_ml.exceptions import RubiconException
 
+if TYPE_CHECKING:
+    from rubicon_ml.client import Project
+    from rubicon_ml.domain import Experiment as ExperimentDomain
+
 
 class Experiment(Base, ArtifactMixin, DataframeMixin, TagMixin):
     """A client experiment.
@@ -30,9 +38,11 @@ class Experiment(Base, ArtifactMixin, DataframeMixin, TagMixin):
         The project that the experiment is logged to.
     """
 
-    def __init__(self, domain, parent):
+    def __init__(self, domain: ExperimentDomain, parent: Project):
         super().__init__(domain, parent._config)
 
+        self._domain: ExperimentDomain
+
         self._parent = parent
         self._artifacts = []
         self._dataframes = []
diff --git a/rubicon_ml/client/feature.py b/rubicon_ml/client/feature.py
index be1991fa..505d2803 100644
--- a/rubicon_ml/client/feature.py
+++ b/rubicon_ml/client/feature.py
@@ -1,5 +1,14 @@
+from __future__ import annotations
+
+from datetime import datetime
+from typing import TYPE_CHECKING, Optional
+
 from rubicon_ml.client import Base, TagMixin
 
+if TYPE_CHECKING:
+    from rubicon_ml.client import Experiment
+    from rubicon_ml.domain import Feature as FeatureDomain
+
 
 class Feature(Base, TagMixin):
     """A client feature.
@@ -25,24 +34,26 @@ class Feature(Base, TagMixin):
         logged to.
     """
 
-    def __init__(self, domain, parent):
+    def __init__(self, domain: FeatureDomain, parent: Experiment):
         super().__init__(domain, parent._config)
 
+        self._domain: FeatureDomain
+
         self._data = None
         self._parent = parent
 
     @property
-    def id(self):
+    def id(self) -> str:
         """Get the feature's id."""
         return self._domain.id
 
     @property
-    def name(self):
+    def name(self) -> Optional[str]:
         """Get the feature's name."""
         return self._domain.name
 
     @property
-    def description(self):
+    def description(self) -> Optional[str]:
         """Get the feature's description."""
         return self._domain.description
 
@@ -52,11 +63,11 @@ def importance(self):
         return self._domain.importance
 
     @property
-    def created_at(self):
+    def created_at(self) -> datetime:
         """Get the feature's created_at."""
         return self._domain.created_at
 
     @property
-    def parent(self):
+    def parent(self) -> Experiment:
         """Get the feature's parent client object."""
         return self._parent
diff --git a/rubicon_ml/client/metric.py b/rubicon_ml/client/metric.py
index 4e61588a..d5390c26 100644
--- a/rubicon_ml/client/metric.py
+++ b/rubicon_ml/client/metric.py
@@ -1,5 +1,14 @@
+from __future__ import annotations
+
+from datetime import datetime
+from typing import TYPE_CHECKING, Optional
+
 from rubicon_ml.client import Base, TagMixin
 
+if TYPE_CHECKING:
+    from rubicon_ml.client import Experiment
+    from rubicon_ml.domain import Metric as MetricDomain
+
 
 class Metric(Base, TagMixin):
     """A client metric.
@@ -21,19 +30,21 @@ class Metric(Base, TagMixin):
         logged to.
     """
 
-    def __init__(self, domain, parent):
+    def __init__(self, domain: MetricDomain, parent: Experiment):
         super().__init__(domain, parent._config)
 
+        self._domain: MetricDomain
+
         self._data = None
         self._parent = parent
 
     @property
-    def id(self):
+    def id(self) -> str:
         """Get the metric's id."""
         return self._domain.id
 
     @property
-    def name(self):
+    def name(self) -> Optional[str]:
         """Get the metric's name."""
         return self._domain.name
 
@@ -43,21 +54,21 @@ def value(self):
         return self._domain.value
 
     @property
-    def directionality(self):
+    def directionality(self) -> str:
         """Get the metric's directionality."""
         return self._domain.directionality
 
     @property
-    def description(self):
+    def description(self) -> Optional[str]:
         """Get the metric's description."""
         return self._domain.description
 
     @property
-    def created_at(self):
+    def created_at(self) -> datetime:
         """Get the metric's created_at."""
         return self._domain.created_at
 
     @property
-    def parent(self):
+    def parent(self) -> Experiment:
         """Get the metric's parent client object."""
         return self._parent
diff --git a/rubicon_ml/client/mixin.py b/rubicon_ml/client/mixin.py
index f1c792ac..37cc5c0e 100644
--- a/rubicon_ml/client/mixin.py
+++ b/rubicon_ml/client/mixin.py
@@ -1,20 +1,33 @@
+from __future__ import annotations
+
 import os
 import pickle
 import subprocess
 import warnings
 from datetime import datetime
+from typing import TYPE_CHECKING, Any, List, Optional, TextIO, Union
 
 import fsspec
 
 from rubicon_ml import client, domain
 from rubicon_ml.client.utils.exception_handling import failsafe
 from rubicon_ml.client.utils.tags import filter_children
+from rubicon_ml.domain import Artifact as ArtifactDomain
 from rubicon_ml.exceptions import RubiconException
 
+if TYPE_CHECKING:
+    import dask.dataframe as dd
+    import pandas as pd
+
+    from rubicon_ml.client import Artifact, Dataframe
+    from rubicon_ml.domain import DOMAIN_TYPES
+
 
 class ArtifactMixin:
     """Adds artifact support to a client object."""
 
+    _domain: ArtifactDomain
+
     def _validate_data(self, data_bytes, data_file, data_object, data_path, name):
         """Raises a `RubiconException` if the data to log as
         an artifact is improperly provided.
@@ -47,14 +60,14 @@ def _validate_data(self, data_bytes, data_file, data_object, data_path, name):
     @failsafe
     def log_artifact(
         self,
-        data_bytes=None,
-        data_file=None,
-        data_object=None,
-        data_path=None,
-        name=None,
-        description=None,
-        tags=[],
-    ):
+        data_bytes: Optional[bytes] = None,
+        data_file: Optional[TextIO] = None,
+        data_object: Optional[Any] = None,
+        data_path: Optional[str] = None,
+        name: Optional[str] = None,
+        description: Optional[str] = None,
+        tags: Optional[List[str]] = None,
+    ) -> Artifact:
         """Log an artifact to this client object.
 
         Parameters
@@ -108,7 +121,8 @@ def log_artifact(
         ...     data_path="./path/to/artifact.pkl", description="log artifact from file path"
         ... )
         """
-
+        if tags is None:
+            tags = []
         if not isinstance(tags, list) or not all([isinstance(tag, str) for tag in tags]):
             raise ValueError("`tags` must be `list` of type `str`")
 
@@ -127,7 +141,7 @@ def log_artifact(
 
         return client.Artifact(artifact, self)
 
-    def _get_environment_bytes(self, export_cmd):
+    def _get_environment_bytes(self, export_cmd: List[str]) -> bytes:
         """Get the working environment as a sequence of bytes.
 
         Parameters
@@ -148,7 +162,7 @@ def _get_environment_bytes(self, export_cmd):
         return completed_process.stdout
 
     @failsafe
-    def log_conda_environment(self, artifact_name=None):
+    def log_conda_environment(self, artifact_name: Optional[str] = None) -> Artifact:
         """Log the conda environment as an artifact to this client object.
         Useful for recreating your exact environment at a later date.
 
@@ -175,7 +189,7 @@ def log_conda_environment(self, artifact_name=None):
         return artifact
 
     @failsafe
-    def log_pip_requirements(self, artifact_name=None):
+    def log_pip_requirements(self, artifact_name: Optional[str] = None) -> Artifact:
         """Log the pip requirements as an artifact to this client object.
         Useful for recreating your exact environment at a later date.
 
@@ -198,7 +212,9 @@ def log_pip_requirements(self, artifact_name=None):
         return artifact
 
     @failsafe
-    def artifacts(self, name=None, tags=[], qtype="or"):
+    def artifacts(
+        self, name: Optional[str] = None, tags: Optional[List[str]] = None, qtype: str = "or"
+    ) -> List[Artifact]:
         """Get the artifacts logged to this client object.
 
         Parameters
@@ -216,6 +232,8 @@ def artifacts(self, name=None, tags=[], qtype="or"):
         list of rubicon.client.Artifact
             The artifacts previously logged to this client object.
         """
+        if tags is None:
+            tags = []
         project_name, experiment_id = self._get_identifiers()
         return_err = None
         for repo in self.repositories:
@@ -233,7 +251,7 @@ def artifacts(self, name=None, tags=[], qtype="or"):
         raise RubiconException("all configured storage backends failed") from return_err
 
     @failsafe
-    def artifact(self, name=None, id=None):
+    def artifact(self, name: Optional[str] = None, id: Optional[str] = None) -> Artifact:
         """Get an artifact logged to this project by id or name.
 
         Parameters
@@ -279,7 +297,7 @@ def artifact(self, name=None, id=None):
         raise RubiconException("all configured storage backends failed") from return_err
 
     @failsafe
-    def delete_artifacts(self, ids):
+    def delete_artifacts(self, ids: List[str]):
         """Delete the artifacts logged to with client object
         with ids `ids`.
 
@@ -298,8 +316,16 @@ def delete_artifacts(self, ids):
 class DataframeMixin:
     """Adds dataframe support to a client object."""
 
+    _domain: DOMAIN_TYPES
+
     @failsafe
-    def log_dataframe(self, df, description=None, name=None, tags=[]):
+    def log_dataframe(
+        self,
+        df: Union[pd.DataFrame, dd.DataFrame],
+        description: Optional[str] = None,
+        name: Optional[str] = None,
+        tags: Optional[List[str]] = None,
+    ) -> Dataframe:
         """Log a dataframe to this client object.
 
         Parameters
@@ -317,6 +343,8 @@ def log_dataframe(self, df, description=None, name=None, tags=[]):
         rubicon.client.Dataframe
             The new dataframe.
         """
+        if tags is None:
+            tags = []
         if not isinstance(tags, list) or not all([isinstance(tag, str) for tag in tags]):
             raise ValueError("`tags` must be `list` of type `str`")
 
@@ -334,7 +362,9 @@ def log_dataframe(self, df, description=None, name=None, tags=[]):
         return client.Dataframe(dataframe, self)
 
     @failsafe
-    def dataframes(self, name=None, tags=[], qtype="or"):
+    def dataframes(
+        self, name: Optional[str] = None, tags: Optional[List[str]] = None, qtype: str = "or"
+    ) -> List[Dataframe]:
         """Get the dataframes logged to this client object.
 
         Parameters
@@ -352,6 +382,8 @@ def dataframes(self, name=None, tags=[], qtype="or"):
         list of rubicon.client.Dataframe
             The dataframes previously logged to this client object.
         """
+        if tags is None:
+            tags = []
         project_name, experiment_id = self._get_identifiers()
         return_err = None
         for repo in self.repositories:
@@ -369,7 +401,7 @@ def dataframes(self, name=None, tags=[], qtype="or"):
         raise RubiconException("all configured storage backends failed") from return_err
 
     @failsafe
-    def dataframe(self, name=None, id=None):
+    def dataframe(self, name: Optional[str] = None, id: Optional[str] = None) -> Dataframe:
         """
         Get the dataframe logged to this client object.
 
@@ -419,7 +451,7 @@ def dataframe(self, name=None, id=None):
         raise RubiconException("all configured storage backends failed") from return_err
 
     @failsafe
-    def delete_dataframes(self, ids):
+    def delete_dataframes(self, ids: List[str]):
         """Delete the dataframes with ids `ids` logged to
         this client object.
 
@@ -438,6 +470,8 @@ def delete_dataframes(self, ids):
 class TagMixin:
     """Adds tag support to a client object."""
 
+    _domain: DOMAIN_TYPES
+
     def _get_taggable_identifiers(self):
         project_name, experiment_id = self._parent._get_identifiers()
         entity_identifier = None
@@ -455,7 +489,7 @@ def _get_taggable_identifiers(self):
         return project_name, experiment_id, entity_identifier
 
     @failsafe
-    def add_tags(self, tags):
+    def add_tags(self, tags: List[str]):
         """Add tags to this client object.
 
         Parameters
@@ -479,7 +513,7 @@ def add_tags(self, tags):
             )
 
     @failsafe
-    def remove_tags(self, tags):
+    def remove_tags(self, tags: List[str]):
         """Remove tags from this client object.
 
         Parameters
@@ -508,7 +542,7 @@ def _update_tags(self, tag_data):
             self._domain.remove_tags(tag.get("removed_tags", []))
 
     @property
-    def tags(self):
+    def tags(self) -> List[str]:
         """Get this client object's tags."""
         project_name, experiment_id, entity_identifier = self._get_taggable_identifiers()
         return_err = None
diff --git a/rubicon_ml/client/parameter.py b/rubicon_ml/client/parameter.py
index ada71e34..e17a20ad 100644
--- a/rubicon_ml/client/parameter.py
+++ b/rubicon_ml/client/parameter.py
@@ -1,5 +1,14 @@
+from __future__ import annotations
+
+from datetime import datetime
+from typing import TYPE_CHECKING, Optional, Union
+
 from rubicon_ml.client import Base, TagMixin
 
+if TYPE_CHECKING:
+    from rubicon_ml.client import Experiment
+    from rubicon_ml.domain import Parameter as ParameterDomain
+
 
 class Parameter(Base, TagMixin):
     """A client parameter.
@@ -22,36 +31,38 @@ class Parameter(Base, TagMixin):
         The experiment that the parameter is logged to.
     """
 
-    def __init__(self, domain, parent):
+    def __init__(self, domain: ParameterDomain, parent: Experiment):
         super().__init__(domain, parent._config)
         self._parent = parent
 
+        self._domain: ParameterDomain
+
     @property
-    def id(self):
+    def id(self) -> str:
         """Get the parameter's id."""
         return self._domain.id
 
     @property
-    def name(self):
+    def name(self) -> Optional[str]:
         """Get the parameter's name."""
         return self._domain.name
 
     @property
-    def value(self):
+    def value(self) -> Optional[Union[object, float]]:
         """Get the parameter's value."""
         return self._domain.value
 
     @property
-    def description(self):
+    def description(self) -> Optional[str]:
         """Get the parameter's description."""
         return self._domain.description
 
     @property
-    def created_at(self):
+    def created_at(self) -> datetime:
         """Get the time the parameter was created."""
         return self._domain.created_at
 
     @property
-    def parent(self):
+    def parent(self) -> Experiment:
         """Get the parameter's parent client object."""
         return self._parent
diff --git a/rubicon_ml/client/project.py b/rubicon_ml/client/project.py
index 8ed592f9..dffc5cb2 100644
--- a/rubicon_ml/client/project.py
+++ b/rubicon_ml/client/project.py
@@ -1,6 +1,8 @@
+from __future__ import annotations
+
 import subprocess
 import warnings
-from typing import List, Optional
+from typing import TYPE_CHECKING, Dict, List, Optional, Tuple, Union
 
 import dask.dataframe as dd
 import pandas as pd
@@ -11,6 +13,11 @@
 from rubicon_ml.client.utils.tags import filter_children
 from rubicon_ml.exceptions import RubiconException
 
+if TYPE_CHECKING:
+    from rubicon_ml import Rubicon
+    from rubicon_ml.client import Config, Dataframe
+    from rubicon_ml.domain import Project as ProjectDomain
+
 
 class Project(Base, ArtifactMixin, DataframeMixin):
     """A client project.
@@ -26,14 +33,16 @@ class Project(Base, ArtifactMixin, DataframeMixin):
         The config, which specifies the underlying repository.
     """
 
-    def __init__(self, domain, config=None):
+    def __init__(self, domain: ProjectDomain, config: Optional[Config] = None):
         super().__init__(domain, config)
 
+        self._domain: ProjectDomain
+
         self._artifacts = []
         self._dataframes = []
         self._experiments = []
 
-    def _get_branch_name(self):
+    def _get_branch_name(self) -> str:
         """Returns the name of the active branch of the `git` repo
         it is called from.
         """
@@ -42,7 +51,7 @@ def _get_branch_name(self):
 
         return completed_process.stdout.decode("utf8").replace("\n", "")
 
-    def _get_commit_hash(self):
+    def _get_commit_hash(self) -> str:
         """Returns the hash of the last commit to the active branch
         of the `git` repo it is called from.
         """
@@ -51,7 +60,7 @@ def _get_commit_hash(self):
 
         return completed_process.stdout.decode("utf8").replace("\n", "")
 
-    def _get_identifiers(self):
+    def _get_identifiers(self) -> Tuple[Optional[str], None]:
         """Get the project's name."""
         return self.name, None
 
@@ -86,7 +95,7 @@ def _create_experiment_domain(
             tags=tags,
         )
 
-    def _group_experiments(self, experiments, group_by=None):
+    def _group_experiments(self, experiments: List[Experiment], group_by: Optional[str] = None):
         """Groups experiments by `group_by`. Valid options include ["commit_hash"].
 
         Returns
@@ -111,7 +120,7 @@ def _group_experiments(self, experiments, group_by=None):
 
         return grouped_experiments
 
-    def to_dask_df(self, group_by=None):
+    def to_dask_df(self, group_by: Optional[str] = None):
         """DEPRECATED: Available for backwards compatibility."""
         warnings.warn(
             "`to_dask_df` is deprecated and will be removed in a future release. "
@@ -122,7 +131,9 @@ def to_dask_df(self, group_by=None):
         return self.to_df(df_type="dask", group_by=group_by)
 
     @failsafe
-    def to_df(self, df_type="pandas", group_by=None):
+    def to_df(
+        self, df_type: str = "pandas", group_by: Optional[str] = None
+    ) -> Union[pd.DataFrame, Dict[str, pd.DataFrame], dd.DataFrame, Dict[str, dd.DataFrame]]:
         """Loads the project's data into dask or pandas dataframe(s) sorted by
         `created_at`. This includes the experiment details along with parameters
         and metrics.
@@ -138,9 +149,9 @@ def to_df(self, df_type="pandas", group_by=None):
 
         Returns
         -------
-        pandas.DataFrame or list of pandas.DataFrame or dask.DataFrame or list of dask.DataFrame
+        pandas.DataFrame or dict of pandas.DataFrame or dask.DataFrame or dict of dask.DataFrame
             If `group_by` is `None`, a dask or pandas dataframe holding the project's
-            data. Otherwise a list of dask or pandas dataframes holding the project's
+            data. Otherwise a dict of dask or pandas dataframes holding the project's
             data grouped by `group_by`.
         """
         DEFAULT_COLUMNS = [
@@ -199,14 +210,14 @@ def to_df(self, df_type="pandas", group_by=None):
     @failsafe
     def log_experiment(
         self,
-        name=None,
-        description=None,
-        model_name=None,
-        branch_name=None,
-        commit_hash=None,
-        training_metadata=None,
-        tags=[],
-    ):
+        name: Optional[str] = None,
+        description: Optional[str] = None,
+        model_name: Optional[str] = None,
+        branch_name: Optional[str] = None,
+        commit_hash: Optional[str] = None,
+        training_metadata: Optional[Union[Tuple, List[Tuple]]] = None,
+        tags: Optional[List[str]] = None,
+    ) -> Experiment:
         """Log a new experiment to this project.
 
         Parameters
@@ -242,6 +253,8 @@ def log_experiment(
         rubicon.client.Experiment
             The created experiment.
         """
+        if tags is None:
+            tags = []
         if not isinstance(tags, list) or not all([isinstance(tag, str) for tag in tags]):
             raise ValueError("`tags` must be `list` of type `str`")
 
@@ -260,7 +273,7 @@ def log_experiment(
         return Experiment(experiment, self)
 
     @failsafe
-    def experiment(self, id=None, name=None):
+    def experiment(self, id: Optional[str] = None, name: Optional[str] = None) -> Experiment:
         """Get an experiment logged to this project by id or name.
 
         Parameters
@@ -303,7 +316,9 @@ def experiment(self, id=None, name=None):
             raise RubiconException("all configured storage backends failed") from return_err
 
     @failsafe
-    def experiments(self, tags=[], qtype="or", name=None):
+    def experiments(
+        self, tags: Optional[List[str]] = None, qtype: str = "or", name: Optional[str] = None
+    ) -> List[Experiment]:
         """Get the experiments logged to this project.
 
         Parameters
@@ -321,6 +336,8 @@ def experiments(self, tags=[], qtype="or", name=None):
         list of rubicon.client.Experiment
             The experiments previously logged to this project.
         """
+        if tags is None:
+            tags = []
         return_err = None
         for repo in self.repositories:
             try:
@@ -334,7 +351,13 @@ def experiments(self, tags=[], qtype="or", name=None):
         raise RubiconException("all configured storage backends failed") from return_err
 
     @failsafe
-    def dataframes(self, tags=[], qtype="or", recursive=False, name=None):
+    def dataframes(
+        self,
+        tags: Optional[List[str]] = None,
+        qtype: str = "or",
+        recursive: bool = False,
+        name: Optional[str] = None,
+    ) -> List[Dataframe]:
         """Get the dataframes logged to this project.
 
         Parameters
@@ -355,6 +378,8 @@ def dataframes(self, tags=[], qtype="or", recursive=False, name=None):
         list of rubicon.client.Dataframe
             The dataframes previously logged to this client object.
         """
+        if tags is None:
+            tags = []
         super().dataframes(tags=tags, qtype=qtype, name=name)
 
         if recursive is True:
@@ -364,7 +389,11 @@ def dataframes(self, tags=[], qtype="or", recursive=False, name=None):
         return self._dataframes
 
     @failsafe
-    def archive(self, experiments: Optional[List[Experiment]] = None, remote_rubicon=None):
+    def archive(
+        self,
+        experiments: Optional[List[Experiment]] = None,
+        remote_rubicon: Optional[Rubicon] = None,
+    ):
         """Archive the experiments logged to this project.
 
         Parameters
diff --git a/rubicon_ml/client/rubicon.py b/rubicon_ml/client/rubicon.py
index ea9f038b..a7c02a88 100644
--- a/rubicon_ml/client/rubicon.py
+++ b/rubicon_ml/client/rubicon.py
@@ -1,9 +1,11 @@
 import subprocess
 import warnings
+from typing import List, Optional, Tuple, Union
 
 from rubicon_ml import domain
 from rubicon_ml.client import Config, Project
 from rubicon_ml.client.utils.exception_handling import failsafe
+from rubicon_ml.domain.utils import TrainingMetadata
 from rubicon_ml.exceptions import RubiconException
 from rubicon_ml.repository.utils import slugify
 
@@ -32,7 +34,11 @@ class Rubicon:
     """
 
     def __init__(
-        self, persistence="filesystem", root_dir=None, auto_git_enabled=False, **storage_options
+        self,
+        persistence: Optional[str] = "filesystem",
+        root_dir: Optional[str] = None,
+        auto_git_enabled: bool = False,
+        **storage_options,
     ):
         self.config = Config(persistence, root_dir, auto_git_enabled, **storage_options)
 
@@ -64,19 +70,27 @@ def _get_github_url(self):
 
         return github_url
 
-    def _create_project_domain(self, name, description, github_url, training_metadata):
+    def _create_project_domain(
+        self,
+        name: str,
+        description: str,
+        github_url: str,
+        training_metadata: Union[List[Tuple], Tuple],
+    ):
         """Instantiates and returns a project domain object."""
         if self.config.is_auto_git_enabled and github_url is None:
             github_url = self._get_github_url()
 
         if training_metadata is not None:
-            training_metadata = domain.utils.TrainingMetadata(training_metadata)
+            training_metadata_class = TrainingMetadata(training_metadata)
+        else:
+            training_metadata_class = None
 
         return domain.Project(
             name,
             description=description,
             github_url=github_url,
-            training_metadata=training_metadata,
+            training_metadata=training_metadata_class,
         )
 
     @failsafe
diff --git a/rubicon_ml/client/utils/exception_handling.py b/rubicon_ml/client/utils/exception_handling.py
index e8fc3835..36407e79 100644
--- a/rubicon_ml/client/utils/exception_handling.py
+++ b/rubicon_ml/client/utils/exception_handling.py
@@ -2,6 +2,7 @@
 import logging
 import traceback
 import warnings
+from typing import Callable, Optional
 
 FAILURE_MODE = "raise"
 FAILURE_MODES = ["log", "raise", "warn"]
@@ -9,7 +10,9 @@
 TRACEBACK_LIMIT = None
 
 
-def set_failure_mode(failure_mode, traceback_chain=False, traceback_limit=None):
+def set_failure_mode(
+    failure_mode: str, traceback_chain: bool = False, traceback_limit: Optional[int] = None
+) -> None:
     """Set the failure mode.
 
     Parameters
@@ -38,7 +41,7 @@ def set_failure_mode(failure_mode, traceback_chain=False, traceback_limit=None):
     TRACEBACK_LIMIT = traceback_limit
 
 
-def failsafe(func):
+def failsafe(func: Callable) -> Callable:
     @functools.wraps(func)
     def wrapper(*args, **kwargs):
         try:
diff --git a/rubicon_ml/client/utils/tags.py b/rubicon_ml/client/utils/tags.py
index a6c62250..bb444bce 100644
--- a/rubicon_ml/client/utils/tags.py
+++ b/rubicon_ml/client/utils/tags.py
@@ -1,4 +1,7 @@
-def has_tag_requirements(tags, required_tags, qtype):
+from typing import List
+
+
+def has_tag_requirements(tags: List[str], required_tags: List[str], qtype: str) -> bool:
     """Returns True if `tags` meets the requirements based on
     the values of `required_tags` and `qtype`. False otherwise.
     """
diff --git a/rubicon_ml/domain/__init__.py b/rubicon_ml/domain/__init__.py
index ad4002c5..7db2ea93 100644
--- a/rubicon_ml/domain/__init__.py
+++ b/rubicon_ml/domain/__init__.py
@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from typing import Union
+
 from rubicon_ml.domain.artifact import Artifact
 from rubicon_ml.domain.dataframe import Dataframe
 from rubicon_ml.domain.experiment import Experiment
@@ -8,4 +10,6 @@
 from rubicon_ml.domain.parameter import Parameter
 from rubicon_ml.domain.project import Project
 
+DOMAIN_TYPES = Union[Artifact, Dataframe, Experiment, Feature, Metric, Parameter, Project]
+
 __all__ = ["Artifact", "Dataframe", "Experiment", "Feature", "Metric", "Parameter", "Project"]
diff --git a/rubicon_ml/domain/artifact.py b/rubicon_ml/domain/artifact.py
index 4a47f59b..14dcf01b 100644
--- a/rubicon_ml/domain/artifact.py
+++ b/rubicon_ml/domain/artifact.py
@@ -2,7 +2,7 @@
 
 from dataclasses import dataclass, field
 from datetime import datetime
-from typing import List
+from typing import List, Optional
 
 from rubicon_ml.domain.mixin import TagMixin
 from rubicon_ml.domain.utils import uuid
@@ -13,8 +13,8 @@ class Artifact(TagMixin):
     name: str
 
     id: str = field(default_factory=uuid.uuid4)
-    description: str = None
+    description: Optional[str] = None
     created_at: datetime = field(default_factory=datetime.utcnow)
     tags: List[str] = field(default_factory=list)
 
-    parent_id: str = None
+    parent_id: Optional[str] = None
diff --git a/rubicon_ml/domain/dataframe.py b/rubicon_ml/domain/dataframe.py
index f8b12b51..e2f085b8 100644
--- a/rubicon_ml/domain/dataframe.py
+++ b/rubicon_ml/domain/dataframe.py
@@ -2,7 +2,7 @@
 
 from dataclasses import dataclass, field
 from datetime import datetime
-from typing import List
+from typing import List, Optional
 
 from rubicon_ml.domain.mixin import TagMixin
 from rubicon_ml.domain.utils import uuid
@@ -11,9 +11,9 @@
 @dataclass
 class Dataframe(TagMixin):
     id: str = field(default_factory=uuid.uuid4)
-    name: str = None
-    description: str = None
+    name: Optional[str] = None
+    description: Optional[str] = None
     tags: List[str] = field(default_factory=list)
     created_at: datetime = field(default_factory=datetime.utcnow)
 
-    parent_id: str = None
+    parent_id: Optional[str] = None
diff --git a/rubicon_ml/domain/experiment.py b/rubicon_ml/domain/experiment.py
index 7c98da2f..d5118b37 100644
--- a/rubicon_ml/domain/experiment.py
+++ b/rubicon_ml/domain/experiment.py
@@ -2,7 +2,7 @@
 
 from dataclasses import dataclass, field
 from datetime import datetime
-from typing import List
+from typing import List, Optional
 
 from rubicon_ml.domain.mixin import TagMixin
 from rubicon_ml.domain.utils import TrainingMetadata, uuid
@@ -13,11 +13,11 @@ class Experiment(TagMixin):
     project_name: str
 
     id: str = field(default_factory=uuid.uuid4)
-    name: str = None
-    description: str = None
-    model_name: str = None
-    branch_name: str = None
-    commit_hash: str = None
-    training_metadata: TrainingMetadata = None
+    name: Optional[str] = None
+    description: Optional[str] = None
+    model_name: Optional[str] = None
+    branch_name: Optional[str] = None
+    commit_hash: Optional[str] = None
+    training_metadata: Optional[TrainingMetadata] = None
     tags: List[str] = field(default_factory=list)
     created_at: datetime = field(default_factory=datetime.utcnow)
diff --git a/rubicon_ml/domain/feature.py b/rubicon_ml/domain/feature.py
index 9da684dd..fd4215d4 100644
--- a/rubicon_ml/domain/feature.py
+++ b/rubicon_ml/domain/feature.py
@@ -1,6 +1,6 @@
 from dataclasses import dataclass, field
 from datetime import datetime
-from typing import List
+from typing import List, Optional
 
 from rubicon_ml.domain.mixin import TagMixin
 from rubicon_ml.domain.utils import uuid
@@ -11,7 +11,7 @@ class Feature(TagMixin):
     name: str
 
     id: str = field(default_factory=uuid.uuid4)
-    description: str = None
-    importance: float = None
+    description: Optional[str] = None
+    importance: Optional[float] = None
     tags: List[str] = field(default_factory=list)
     created_at: datetime = field(default_factory=datetime.utcnow)
diff --git a/rubicon_ml/domain/metric.py b/rubicon_ml/domain/metric.py
index b58278fa..9efd338d 100644
--- a/rubicon_ml/domain/metric.py
+++ b/rubicon_ml/domain/metric.py
@@ -1,6 +1,6 @@
 from dataclasses import dataclass, field
 from datetime import datetime
-from typing import List
+from typing import List, Optional
 
 from rubicon_ml.domain.mixin import TagMixin
 from rubicon_ml.domain.utils import uuid
@@ -14,7 +14,7 @@ class Metric(TagMixin):
     value: float
 
     id: str = field(default_factory=uuid.uuid4)
-    description: str = None
+    description: Optional[str] = None
     directionality: str = "score"
     created_at: datetime = field(default_factory=datetime.utcnow)
     tags: List[str] = field(default_factory=list)
diff --git a/rubicon_ml/domain/mixin.py b/rubicon_ml/domain/mixin.py
index 4dd3f4ab..974f3bdb 100644
--- a/rubicon_ml/domain/mixin.py
+++ b/rubicon_ml/domain/mixin.py
@@ -1,8 +1,27 @@
+from typing import List
+
+
 class TagMixin:
     """Adds tagging support to a domain model."""
 
-    def add_tags(self, tags):
+    def add_tags(self, tags: List[str]):
+        """
+        Add new tags to this model.
+
+        Parameters
+        ----------
+        tags : List[str]
+            A list of string tags to add to the domain model.
+        """
         self.tags = list(set(self.tags).union(set(tags)))
 
-    def remove_tags(self, tags):
+    def remove_tags(self, tags: List[str]):
+        """
+        Remove tags from this model.
+
+        Parameters
+        ----------
+        tags : List[str]
+            A list of string tags to remove from this domain model.
+        """
         self.tags = list(set(self.tags).difference(set(tags)))
diff --git a/rubicon_ml/domain/parameter.py b/rubicon_ml/domain/parameter.py
index f8497416..1238c5cc 100644
--- a/rubicon_ml/domain/parameter.py
+++ b/rubicon_ml/domain/parameter.py
@@ -1,6 +1,6 @@
 from dataclasses import dataclass, field
 from datetime import datetime
-from typing import List
+from typing import List, Optional
 
 from rubicon_ml.domain.mixin import TagMixin
 from rubicon_ml.domain.utils import uuid
@@ -12,6 +12,6 @@ class Parameter(TagMixin):
 
     id: str = field(default_factory=uuid.uuid4)
     value: object = None
-    description: str = None
+    description: Optional[str] = None
     tags: List[str] = field(default_factory=list)
     created_at: datetime = field(default_factory=datetime.utcnow)
diff --git a/rubicon_ml/domain/project.py b/rubicon_ml/domain/project.py
index fe7190b4..ff159590 100644
--- a/rubicon_ml/domain/project.py
+++ b/rubicon_ml/domain/project.py
@@ -2,6 +2,7 @@
 
 from dataclasses import dataclass, field
 from datetime import datetime
+from typing import Optional
 
 from rubicon_ml.domain.utils import TrainingMetadata, uuid
 
@@ -11,7 +12,7 @@ class Project:
     name: str
 
     id: str = field(default_factory=uuid.uuid4)
-    description: str = None
-    github_url: str = None
-    training_metadata: TrainingMetadata = None
+    description: Optional[str] = None
+    github_url: Optional[str] = None
+    training_metadata: Optional[TrainingMetadata] = None
     created_at: datetime = field(default_factory=datetime.utcnow)
diff --git a/rubicon_ml/domain/utils/training_metadata.py b/rubicon_ml/domain/utils/training_metadata.py
index b766b474..a636756b 100644
--- a/rubicon_ml/domain/utils/training_metadata.py
+++ b/rubicon_ml/domain/utils/training_metadata.py
@@ -1,3 +1,5 @@
+from typing import List, Tuple, Union
+
 from rubicon_ml.exceptions import RubiconException
 
 
@@ -25,7 +27,7 @@ class TrainingMetadata:
     [('s3', ['bucket/a.csv', 'bucket/b.csv'], 'SELECT * FROM x')]
     """
 
-    def __init__(self, training_metadata):
+    def __init__(self, training_metadata: Union[List[Tuple], Tuple]):
         if not isinstance(training_metadata, list):
             training_metadata = [training_metadata]
 
@@ -34,5 +36,5 @@ def __init__(self, training_metadata):
 
         self.training_metadata = training_metadata
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return str(self.training_metadata)
diff --git a/rubicon_ml/domain/utils/uuid.py b/rubicon_ml/domain/utils/uuid.py
index c57a60bb..bbf1f567 100644
--- a/rubicon_ml/domain/utils/uuid.py
+++ b/rubicon_ml/domain/utils/uuid.py
@@ -1,7 +1,7 @@
 import uuid
 
 
-def uuid4():
+def uuid4() -> str:
     """Generate a UUID as a string in a single function.
 
     To be used as a default factory within `dataclasses.field`.
diff --git a/rubicon_ml/repository/base.py b/rubicon_ml/repository/base.py
index 9b33a2c1..950cefd7 100644
--- a/rubicon_ml/repository/base.py
+++ b/rubicon_ml/repository/base.py
@@ -36,7 +36,7 @@ class BaseRepository:
         the underlying filesystem class.
     """
 
-    def __init__(self, root_dir, **storage_options):
+    def __init__(self, root_dir: str, **storage_options):
         self.filesystem = fsspec.filesystem(self.PROTOCOL, **storage_options)
         self.root_dir = root_dir.rstrip("/")