diff --git a/README.md b/README.md
index 397a15e..f916599 100644
--- a/README.md
+++ b/README.md
@@ -52,13 +52,14 @@ app.get("/", (req) => {
   schema: {
     query: [
       mustExist("hello"),
+      (query) => query["hello"] !== "world",
     ],
   },
 });
 ```
 
-This schema would only allow requests with the `hello` query (for example,
-`GET /?hello=yes`). The following helper functions are currently available:
+This schema would only allow requests with the `hello` query present and the value not being `"world"` 
+(for example, `GET /?hello=yes`). The following helper functions are currently available:
 
 - `mustExist(key)`
 - `valueMustBeOfType(key, type)`
diff --git a/aqua.ts b/aqua.ts
index 290e6c1..2a2ee89 100644
--- a/aqua.ts
+++ b/aqua.ts
@@ -99,17 +99,10 @@ export interface Options {
   };
 }
 
-export type RoutingSchemaValidationFunction = (
-  this: RoutingSchemaValidationContext,
-  context: RoutingSchemaValidationContext,
-) => boolean;
-
-type BroadestSchemaValidationContextValueTypes = Json;
-
-type RoutingSchemaValidationContext = Record<
-  string,
-  BroadestSchemaValidationContextValueTypes
->;
+export type RoutingSchemaValidationFunction<Context> = (
+  this: Context,
+  context: Context,
+) => boolean | Promise<boolean>;
 
 type RoutingSchemaKeys =
   | "body"
@@ -119,7 +112,9 @@ type RoutingSchemaKeys =
   | "headers";
 
 type RoutingSchema = {
-  [requestKey in RoutingSchemaKeys]?: RoutingSchemaValidationFunction[];
+  [requestKey in RoutingSchemaKeys]?: RoutingSchemaValidationFunction<
+    Request[requestKey]
+  >[];
 };
 
 export interface RoutingOptions {
@@ -131,7 +126,9 @@ export enum MiddlewareType {
   Outgoing = "Outgoing",
 }
 
-export function mustExist(key: string): RoutingSchemaValidationFunction {
+export function mustExist(
+  key: string,
+): RoutingSchemaValidationFunction<Record<string, unknown>> {
   return function () {
     return Object.keys(this).includes(key);
   };
@@ -140,7 +137,7 @@ export function mustExist(key: string): RoutingSchemaValidationFunction {
 export function valueMustBeOfType(
   key: string,
   type: "string" | "number" | "boolean" | "object" | "undefined",
-): RoutingSchemaValidationFunction {
+): RoutingSchemaValidationFunction<Record<string, unknown>> {
   return function () {
     return Object.keys(this).includes(key) && typeof this[key] === type;
   };
@@ -148,8 +145,8 @@ export function valueMustBeOfType(
 
 export function mustContainValue(
   key: string,
-  values: BroadestSchemaValidationContextValueTypes[],
-): RoutingSchemaValidationFunction {
+  values: unknown[],
+): RoutingSchemaValidationFunction<Record<string, unknown>> {
   return function () {
     return Object.keys(this).includes(key) && values.includes(this[key]);
   };
@@ -310,12 +307,12 @@ export default class Aqua {
         ) as RoutingSchemaKeys[]
       ) {
         for (
-          const validationFunction of route.options.schema[
+          const validationFunction of (route.options.schema[
             routingSchemaKey
-          ] || []
+          ] || []) as RoutingSchemaValidationFunction<unknown>[]
         ) {
           const schemaContext = req[routingSchemaKey];
-          if (!validationFunction.bind(schemaContext)(schemaContext)) {
+          if (!(await validationFunction.bind(schemaContext)(schemaContext))) {
             passedAllValidations = false;
             break routingSchemaIterator;
           }
diff --git a/tests/uptests.ts b/tests/uptests.ts
index 7073b88..7463b22 100644
--- a/tests/uptests.ts
+++ b/tests/uptests.ts
@@ -102,9 +102,7 @@ registerTest(
 
     const content = await requestContent(`/api/v2/hello/world/i`);
     if (content !== "Not found.") {
-      throw Error(
-        "URL parameters don't seem to work",
-      );
+      throw Error("URL parameters don't seem to work");
     }
   },
 );
@@ -125,9 +123,7 @@ registerTest(
 
     const content = await requestContent(`/api3/hello/test`);
     if (content === "matched") {
-      throw Error(
-        "URL parameters slash positioning caused an error",
-      );
+      throw Error("URL parameters slash positioning caused an error");
     }
   },
 );
@@ -272,6 +268,56 @@ registerTest("Parameter schemas working?", async () => {
   }
 });
 
+registerTest("Async schema validation functions passes?", async () => {
+  app.get(
+    "/test-parameter-schema-working-async/:hello",
+    (_req) => "Hello, World!",
+    {
+      schema: {
+        parameters: [
+          async (parameters) => {
+            return await new Promise((r) => r(!!parameters["hello"]));
+          },
+        ],
+      },
+    },
+  );
+
+  const content = await requestContent(
+    "/test-parameter-schema-working-async/test",
+  );
+  if (content !== "Hello, World!") {
+    throw Error(
+      "Async schema validation function didn't pass although it shouldn",
+    );
+  }
+});
+
+registerTest("Async schema validation functions fails?", async () => {
+  app.get(
+    "/test-parameter-schema-working-async-fail/:hello",
+    (_req) => "Hello, World!",
+    {
+      schema: {
+        parameters: [
+          async (parameters) => {
+            return await new Promise((r) => r(!!parameters["notfound"]));
+          },
+        ],
+      },
+    },
+  );
+
+  const content = await requestContent(
+    "/test-parameter-schema-working-async-fail/test",
+  );
+  if (content === "Hello, World!") {
+    throw Error(
+      "Async schema validation function passed although it shouldn't",
+    );
+  }
+});
+
 registerTest(
   "Parameter schemas failing if validation functions should return false provided?",
   async () => {