diff --git a/docs/src/utilities.jl b/docs/src/utilities.jl
index c0a3bda..2824775 100644
--- a/docs/src/utilities.jl
+++ b/docs/src/utilities.jl
@@ -14,6 +14,8 @@ A.accessors()
 
 @test length(A.accessors()) == 35 #src
 
+@test length(A.required_functions()) == 11
+
 # You do not need to overload all of them (e.g., if you model does not have any
 # genes you can completely omit all gene-related functions). The main required
 # ones (esp. the reaction- and metabolite-related ones) will throw an error if
diff --git a/src/AbstractFBCModels.jl b/src/AbstractFBCModels.jl
index 98f3e94..b51c643 100644
--- a/src/AbstractFBCModels.jl
+++ b/src/AbstractFBCModels.jl
@@ -30,9 +30,9 @@ module AbstractFBCModels
 using DocStringExtensions
 
 include("types.jl")
+include("utils.jl")
 include("accessors.jl")
 include("io.jl")
-include("utils.jl")
 include("testing.jl")
 include("canonical.jl")
 
diff --git a/src/accessors.jl b/src/accessors.jl
index 0e6ccc5..96f978a 100644
--- a/src/accessors.jl
+++ b/src/accessors.jl
@@ -10,7 +10,7 @@ modeling, such as metabolite exchanges, separated forward and reverse reaction
 directions, supplies of enzymatic and genetic material and virtual cell volume,
 etc.
 """
-reactions(a::AbstractFBCModel)::Vector{String} = unimplemented(typeof(a), :reactions)
+@required reactions(a::AbstractFBCModel)::Vector{String}
 
 """
 $(TYPEDSIGNATURES)
@@ -19,7 +19,7 @@ The number of reactions in the given model. Must be equal to the length of the
 vector returned by [`reactions`](@ref), and may be more efficient for just
 determining the size.
 """
-n_reactions(a::AbstractFBCModel)::Int = unimplemented(typeof(a), :n_reactions)
+@required n_reactions(a::AbstractFBCModel)::Int
 
 """
 $(TYPEDSIGNATURES)
@@ -29,7 +29,7 @@ Return a vector of metabolite identifiers in a model.
 As with [`reactions`](@ref), some metabolites in models may be virtual,
 representing purely technical equality constraints.
 """
-metabolites(a::AbstractFBCModel)::Vector{String} = unimplemented(typeof(a), :metabolites)
+@required metabolites(a::AbstractFBCModel)::Vector{String}
 
 """
 $(TYPEDSIGNATURES)
@@ -39,7 +39,7 @@ the vector returned by [`metabolites`](@ref), and may be more efficient for
 just determining the size.
 """
 function n_metabolites end
-n_metabolites(a::AbstractFBCModel)::Int = unimplemented(typeof(a), :n_metabolites)
+@required n_metabolites(a::AbstractFBCModel)::Int
 
 """
 $(TYPEDSIGNATURES)
@@ -49,7 +49,7 @@ Return identifiers of all genes contained in the model. Empty if none.
 Genes are also sometimes called "gene products" but we write genes for
 simplicity.
 """
-genes(a::AbstractFBCModel)::Vector{String} = unimplemented(typeof(a), :genes)
+@required genes(a::AbstractFBCModel)::Vector{String}
 
 """
 $(TYPEDSIGNATURES)
@@ -60,7 +60,7 @@ by [`genes`](@ref)).
 This may be more efficient than calling [`genes`](@ref) and measuring the
 array.
 """
-n_genes(a::AbstractFBCModel)::Int = unimplemented(typeof(a), :n_genes)
+@required n_genes(a::AbstractFBCModel)::Int
 
 """
 $(TYPEDSIGNATURES)
@@ -94,7 +94,7 @@ The sparse stoichiometric matrix of a given model.
 This usually corresponds to all the equality constraints in the model. The
 matrix must be of size [`n_metabolites`](@ref) by [`n_reactions`](@ref).
 """
-stoichiometry(a::AbstractFBCModel)::SparseMat = unimplemented(typeof(a), :stoichiometry)
+@required stoichiometry(a::AbstractFBCModel)::SparseMat
 
 """
 $(TYPEDSIGNATURES)
@@ -112,8 +112,7 @@ $(TYPEDSIGNATURES)
 
 Lower and upper bounds of all reactions in the model.
 """
-bounds(a::AbstractFBCModel)::Tuple{Vector{Float64},Vector{Float64}} =
-    unimplemented(typeof(a), :bounds)
+@required bounds(a::AbstractFBCModel)::Tuple{Vector{Float64},Vector{Float64}}
 
 """
 $(TYPEDSIGNATURES)
@@ -138,7 +137,7 @@ $(TYPEDSIGNATURES)
 
 The objective vector of the model.
 """
-objective(a::AbstractFBCModel)::SparseVec = unimplemented(typeof(a), :objective)
+@required objective(a::AbstractFBCModel)::SparseVec
 
 """
 $(TYPEDSIGNATURES)
@@ -189,8 +188,7 @@ maps the metabolite IDs to their stoichiometric coefficients.
 Using this function may be more efficient in cases than loading the whole
 [`stoichiometry`](@ref).
 """
-reaction_stoichiometry(a::AbstractFBCModel, reaction_id::String)::Dict{String,Float64} =
-    unimplemented(typeof(a), :reaction_stoichiometry)
+@required reaction_stoichiometry(a::AbstractFBCModel, reaction_id::String)::Dict{String,Float64}
 
 """
 $(TYPEDSIGNATURES)
diff --git a/src/io.jl b/src/io.jl
index 343539e..4285c0d 100644
--- a/src/io.jl
+++ b/src/io.jl
@@ -13,7 +13,7 @@ $(TYPEDSIGNATURES)
 
 Save a model to the given path.
 """
-save(a::AbstractFBCModel, path::String)::Nothing = unimplemented(typeof(a), :save)
+@required save(a::AbstractFBCModel, path::String)
 
 """
 $(TYPEDSIGNATURES)
diff --git a/src/utils.jl b/src/utils.jl
index bc4a10d..9e62b7b 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -3,6 +3,26 @@ import Downloads: download
 import SHA: sha256
 import InteractiveUtils: methodswith
 
+
+const REQUIRED_FUNCTIONS = Function[]
+macro required(sig)
+    call_ex = sig.head == :(::) ? sig.args[1] : sig
+    call_ex.head == :call || error("malformed signature definition")
+    name = call_ex.args[1]
+    name isa Symbol || error("malformed signature definition")
+    
+    model_arg_ex = call_ex.args[2]
+    model_arg_ex.head == :(::) || error("malformed signature definition")
+    model_arg = model_arg_ex.args[1]
+    model_arg isa Symbol || error("malformed signature definition")
+
+    return esc(quote
+        Base.@__doc__ $call_ex = $unimplemented(typeof($model_arg), $(Meta.quot(name))) 
+        push!(REQUIRED_FUNCTIONS, $name)
+        $name
+    end)
+end
+
 unimplemented(t::Type, x::Symbol) =
     error("AbstractFBCModels interface method $x is not implemented for type $t")
 
@@ -10,11 +30,13 @@ unimplemented(t::Type, x::Symbol) =
 $(TYPEDSIGNATURES)
 
 Provide a `methodswith`-style listing of accessors that the model implementors
-should implement.
+may implement.
 
 For typesystem reasons, the list **will not contain** methods for
 [`save`](@ref) and [`filename_extensions`](@ref) that dispatch on type objects.
 You should implement these as well.
+
+See also [`required_functions`](@ref) for the minimal list that must be implemented.
 """
 function accessors()
     ms = Method[]
@@ -27,6 +49,30 @@ function accessors()
     return ms
 end
 
+"""
+$(TYPEDSIGNATURES)
+
+Provide a `methodswith`-style listing of functions that the model implementors
+must implement to have a functional `AbstractFBCModel`.
+constrast this to the longer list of items that are returned by [`accessors`](@ref).
+The extra elements have sensible defaults based on these required functions.
+Where-as not defining these methods will result in errors.
+Though depending on your models capability relying on those defaults may mean some
+functionality is hidden.
+(e.g. default [`coupling`](@ref) if you don't implement that is to assume none)
+
+"""
+function required_functions()
+    ms = Method[]
+    for f = REQUIRED_FUNCTIONS
+        methodswith(AbstractFBCModels.AbstractFBCModel, f, ms)
+    end
+    return ms
+end
+
+
+
+
 """
 $(TYPEDSIGNATURES)