diff --git a/packages/authx/src/Config.ts b/packages/authx/src/Config.ts
index fe7d2ddc..4ac52565 100644
--- a/packages/authx/src/Config.ts
+++ b/packages/authx/src/Config.ts
@@ -28,6 +28,7 @@ export interface Config {
     readonly from?: string;
   }) => Promise<any>;
   readonly processSchema?: (schema: GraphQLSchema) => GraphQLSchema;
+  readonly maxRequestsPerMinute: number | null;
 }
 
 export function assertConfig(config: Config): void {
diff --git a/packages/authx/src/Context.ts b/packages/authx/src/Context.ts
index 6e4044ac..bc34862d 100644
--- a/packages/authx/src/Context.ts
+++ b/packages/authx/src/Context.ts
@@ -2,6 +2,7 @@ import { Pool } from "pg";
 import { Authorization } from "./model";
 import { Explanation } from "./util/explanations";
 import { ReadonlyDataLoaderExecutor } from "./loader";
+import { RateLimiter } from "./util/ratelimiter";
 
 export interface Context {
   readonly realm: string;
@@ -10,6 +11,7 @@ export interface Context {
   readonly publicKeys: ReadonlyArray<string>;
   readonly codeValidityDuration: number;
   readonly jwtValidityDuration: number;
+  readonly rateLimiter: RateLimiter;
   readonly sendMail: (options: {
     readonly to: string;
     readonly subject: string;
diff --git a/packages/authx/src/errors.ts b/packages/authx/src/errors.ts
index 5714d9aa..b8161756 100644
--- a/packages/authx/src/errors.ts
+++ b/packages/authx/src/errors.ts
@@ -171,3 +171,25 @@ export class NotImplementedError extends Error {
     this.status = this.statusCode = 501;
   }
 }
+
+export class TooManyRequests extends Error {
+  public fileName?: string;
+  public lineNumber?: number;
+  public status: 429 = 429;
+  public statusCode: 429 = 429;
+  public expose: boolean;
+
+  public constructor(
+    message?: string,
+    expose: boolean = true,
+    fileName?: string,
+    lineNumber?: number
+  ) {
+    super(message);
+    if (typeof fileName !== undefined) this.fileName = fileName;
+    if (typeof lineNumber !== undefined) this.lineNumber = lineNumber;
+    this.message = message || "Too many requests.";
+    this.expose = expose;
+    this.status = this.statusCode = 429;
+  }
+}
diff --git a/packages/authx/src/graphql/GraphQLAuthorization.ts b/packages/authx/src/graphql/GraphQLAuthorization.ts
index 35b72306..8a2cf100 100644
--- a/packages/authx/src/graphql/GraphQLAuthorization.ts
+++ b/packages/authx/src/graphql/GraphQLAuthorization.ts
@@ -148,6 +148,7 @@ export const GraphQLAuthorization: GraphQLObjectType<
           privateKey,
           authorization: a,
           executor,
+          rateLimiter,
         }: Context
       ): Promise<null | string> {
         if (!a) return null;
@@ -174,6 +175,8 @@ export const GraphQLAuthorization: GraphQLObjectType<
         }
 
         if (args.format === "bearer") {
+          rateLimiter.limit(authorization.id);
+
           const tokenId = v4();
           const grant = await authorization.grant(executor);
           await authorization.invoke(executor, {
diff --git a/packages/authx/src/index.ts b/packages/authx/src/index.ts
index 7a96e09b..0967eae9 100644
--- a/packages/authx/src/index.ts
+++ b/packages/authx/src/index.ts
@@ -17,6 +17,11 @@ import { StrategyCollection } from "./StrategyCollection";
 import { UnsupportedMediaTypeError } from "./errors";
 import { createAuthXExplanations } from "./explanations";
 import { DataLoaderExecutor } from "./loader";
+import {
+  LocalMemoryRateLimiter,
+  NoOpRateLimiter,
+  RateLimiter,
+} from "./util/ratelimiter";
 
 export * from "./x";
 export * from "./errors";
@@ -33,6 +38,7 @@ type AuthXMiddleware = Middleware<any, KoaContext & { [x]: Context }>;
 
 export class AuthX extends Router<any, { [x]: Context }> {
   public readonly pool: Pool;
+  public readonly rateLimiter: RateLimiter;
   public constructor(config: Config & IRouterOptions) {
     assertConfig(config);
     super(config);
@@ -46,6 +52,10 @@ export class AuthX extends Router<any, { [x]: Context }> {
 
     // create a database pool
     this.pool = new Pool(config.pg);
+    this.rateLimiter =
+      typeof config.maxRequestsPerMinute === "number"
+        ? new LocalMemoryRateLimiter(config.maxRequestsPerMinute)
+        : new NoOpRateLimiter();
 
     // define the context middleware
     const contextMiddleware = async (
@@ -69,6 +79,8 @@ export class AuthX extends Router<any, { [x]: Context }> {
         if (basic) {
           authorization = await fromBasic(tx, basic);
 
+          this.rateLimiter.limit(authorization.id);
+
           // Invoke the authorization. Because the resource validates basic
           // tokens by making a GraphQL request here, each request can be
           // considered an invocation.
@@ -105,6 +117,7 @@ export class AuthX extends Router<any, { [x]: Context }> {
           authorization,
           explanations: explanations,
           executor: new DataLoaderExecutor(this.pool, strategies),
+          rateLimiter: this.rateLimiter,
         };
 
         ctx[x] = context;
diff --git a/packages/authx/src/oauth2.ts b/packages/authx/src/oauth2.ts
index 44a433a6..d7900179 100644
--- a/packages/authx/src/oauth2.ts
+++ b/packages/authx/src/oauth2.ts
@@ -362,6 +362,8 @@ async function oAuth2Middleware(
             );
           }
 
+          ctx[x].rateLimiter.limit(paramsClientId);
+
           // Authenticate the client with its secret.
           let client;
           try {
@@ -659,6 +661,8 @@ async function oAuth2Middleware(
             );
           }
 
+          ctx[x].rateLimiter.limit(paramsClientId);
+
           const requestedScopeTemplates = paramsScope
             ? paramsScope.split(" ")
             : [];
diff --git a/packages/authx/src/util/ratelimiter.ts b/packages/authx/src/util/ratelimiter.ts
new file mode 100644
index 00000000..e7771231
--- /dev/null
+++ b/packages/authx/src/util/ratelimiter.ts
@@ -0,0 +1,64 @@
+import { performance } from "perf_hooks";
+import { TooManyRequests } from "../errors";
+
+/**
+ * Applies a simple in-memory rate limiting scheme. This system is designed to prevent a single
+ * malfunctioning client from bringing down the service. This system is only designed to prevent
+ * unintentional abuse by malfunctioning clients.
+ */
+export interface RateLimiter {
+  /**
+   * Applies the configured rate limit to the given key. If the key is being excessively used, throws a TooManyRequests exception.
+   * @param key A string representing the key to use for rate limiting. This is generally the id of an authorization,
+   *            client, or credential.
+   */
+  limit(key: string): void;
+}
+
+export class LocalMemoryRateLimiter implements RateLimiter {
+  private readonly map: { [key: string]: number[] } = {
+    __proto__: null as any,
+  };
+
+  constructor(
+    private readonly limitPerWindow = 100,
+    private readonly window = 60 * 1_000,
+    private readonly timeSource: () => number = performance.now
+  ) {}
+
+  limit(key: string): void {
+    const currentTime = this.timeSource();
+
+    for (const existingKey of Object.keys(this.map)) {
+      for (let i = 0; i < this.map[existingKey].length; ++i) {
+        if (currentTime - this.map[existingKey][i] > this.window) {
+          this.map[existingKey].splice(i, 1);
+          --i;
+        }
+      }
+
+      if (this.map[existingKey].length == 0) {
+        delete this.map[existingKey];
+      }
+    }
+
+    if (
+      typeof this.map[key] !== "undefined" &&
+      this.map[key].length >= this.limitPerWindow
+    ) {
+      throw new TooManyRequests();
+    }
+
+    if (typeof this.map[key] === "undefined") {
+      this.map[key] = [];
+    }
+
+    this.map[key].push(currentTime);
+  }
+}
+
+export class NoOpRateLimiter implements RateLimiter {
+  limit(): void {
+    // This rate limiter does nothing. It only exists so that clients can call some rate limiter.
+  }
+}
diff --git a/packages/authx/src/util/raterlimiter.test.ts b/packages/authx/src/util/raterlimiter.test.ts
new file mode 100644
index 00000000..5a0a97c4
--- /dev/null
+++ b/packages/authx/src/util/raterlimiter.test.ts
@@ -0,0 +1,32 @@
+import test from "ava";
+import { LocalMemoryRateLimiter } from "./ratelimiter";
+
+test("Rate limiter over rate", async (t) => {
+  let curTime = 0;
+
+  const limiter = new LocalMemoryRateLimiter(3, 60 * 1_000, () => {
+    curTime += 14_000;
+    return curTime;
+  });
+
+  limiter.limit("A");
+  limiter.limit("B");
+  limiter.limit("C");
+  limiter.limit("A");
+  limiter.limit("B");
+  limiter.limit("C");
+  limiter.limit("A");
+  limiter.limit("B");
+  limiter.limit("C");
+
+  limiter.limit("A");
+  limiter.limit("A");
+  limiter.limit("A");
+
+  try {
+    limiter.limit("A");
+    t.fail("4th call in the same minute should cause 429");
+  } catch (ex) {
+    t.pass();
+  }
+});
diff --git a/packages/strategy-email/src/server/graphql/mutation/authenticateEmail.ts b/packages/strategy-email/src/server/graphql/mutation/authenticateEmail.ts
index 59ea614d..c9845cc1 100644
--- a/packages/strategy-email/src/server/graphql/mutation/authenticateEmail.ts
+++ b/packages/strategy-email/src/server/graphql/mutation/authenticateEmail.ts
@@ -176,6 +176,8 @@ export const authenticateEmail: GraphQLFieldConfig<
         );
       }
 
+      context.rateLimiter.limit(credential.id);
+
       // Invoke the credential.
       await credential.invoke(executor, {
         id: v4(),
diff --git a/packages/strategy-openid/src/server/graphql/mutation/authenticateOpenId.ts b/packages/strategy-openid/src/server/graphql/mutation/authenticateOpenId.ts
index 540451bf..c464977d 100644
--- a/packages/strategy-openid/src/server/graphql/mutation/authenticateOpenId.ts
+++ b/packages/strategy-openid/src/server/graphql/mutation/authenticateOpenId.ts
@@ -290,6 +290,8 @@ export const authenticateOpenId: GraphQLFieldConfig<
         throw new AuthenticationError("No such credential exists.");
       }
 
+      context.rateLimiter.limit(credential.id);
+
       // Invoke the credential.
       await credential.invoke(executor, {
         id: v4(),
diff --git a/packages/strategy-password/src/server/graphql/mutation/authenticatePassword.ts b/packages/strategy-password/src/server/graphql/mutation/authenticatePassword.ts
index c2583b2a..b66d6b2e 100644
--- a/packages/strategy-password/src/server/graphql/mutation/authenticatePassword.ts
+++ b/packages/strategy-password/src/server/graphql/mutation/authenticatePassword.ts
@@ -143,6 +143,8 @@ export const authenticatePassword: GraphQLFieldConfig<
         );
       }
 
+      context.rateLimiter.limit(credential.id);
+
       // Invoke the credential.
       await credential.invoke(executor, {
         id: v4(),
diff --git a/packages/strategy-saml/src/server/graphql/mutation/authenticateSaml.ts b/packages/strategy-saml/src/server/graphql/mutation/authenticateSaml.ts
index ec241c3c..8e4e3727 100644
--- a/packages/strategy-saml/src/server/graphql/mutation/authenticateSaml.ts
+++ b/packages/strategy-saml/src/server/graphql/mutation/authenticateSaml.ts
@@ -321,6 +321,8 @@ export const authenticateSaml: GraphQLFieldConfig<
         );
       }
 
+      context.rateLimiter.limit(credential.id);
+
       // Invoke the credential.
       await credential.invoke(executor, {
         id: v4(),
diff --git a/src/server.ts b/src/server.ts
index da2fcf74..224db564 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -80,6 +80,9 @@ Bac/x5qiUn5fh2xM+wIDAQAB
         ssl: process.env.PGSSL === "true" ? true : false,
         user: process.env.PGUSER ?? undefined,
       },
+      maxRequestsPerMinute: process.env.MAX_REQUESTS_PER_KEY_PER_MINUTE
+        ? parseFloat(process.env.MAX_REQUESTS_PER_KEY_PER_MINUTE)
+        : null,
     });
 
     // Apply the AuthX routes to the app.
diff --git a/src/setup.ts b/src/setup.ts
index 0a97b246..ce43e907 100644
--- a/src/setup.ts
+++ b/src/setup.ts
@@ -127,6 +127,7 @@ Bac/x5qiUn5fh2xM+wIDAQAB
       ssl: process.env.PGSSL === "true" ? true : false,
       user: process.env.PGUSER ?? undefined,
     },
+    maxRequestsPerMinute: null,
   });
 
   // Apply the AuthX routes to the app.