diff --git a/express-api/src/constants/errors.ts b/express-api/src/constants/errors.ts
new file mode 100644
index 000000000..892bcf5a8
--- /dev/null
+++ b/express-api/src/constants/errors.ts
@@ -0,0 +1,20 @@
+import { ErrorWithCode } from '@/utilities/customErrors/ErrorWithCode';
+
+// 4xx Error Codes
+export const BadRequest400 = new ErrorWithCode('Bad request.', 400);
+
+export const Unauthorized401 = new ErrorWithCode('User unauthorized.', 401);
+
+export const Forbidden403 = new ErrorWithCode('Forbidden request.', 403);
+
+export const NotFound404 = new ErrorWithCode('Resource not found.', 404);
+
+export const EndpointNotFound404 = new ErrorWithCode('Requested endpoint not found', 404);
+
+export const NotAllowed405 = new ErrorWithCode('Method not allowed.', 405);
+
+// 5xx Error Codes
+export const ServerError500 = new ErrorWithCode(
+  'The server has encountered a situation it does not know how to handle.',
+  500,
+);
diff --git a/express-api/src/constants/index.ts b/express-api/src/constants/index.ts
index 47c111f17..25344efdf 100644
--- a/express-api/src/constants/index.ts
+++ b/express-api/src/constants/index.ts
@@ -1,10 +1,12 @@
 import networking from '@/constants/networking';
 import switches from '@/constants/switches';
 import urls from '@/constants/urls';
+import * as errors from '@/constants/errors';
 
 const constants = {
   ...networking,
   ...switches,
   ...urls,
+  ...errors,
 };
 export default constants;
diff --git a/express-api/src/express.ts b/express-api/src/express.ts
index 29d9a551b..ad8a4b522 100644
--- a/express-api/src/express.ts
+++ b/express-api/src/express.ts
@@ -12,6 +12,8 @@ import { KEYCLOAK_OPTIONS } from '@/middleware/keycloak/keycloakOptions';
 import swaggerUi from 'swagger-ui-express';
 import { Roles } from '@/constants/roles';
 import swaggerJSON from '@/swagger/swagger-output.json';
+import errorHandler from '@/middleware/errorHandler';
+import { EndpointNotFound404 } from '@/constants/errors';
 
 const app: Application = express();
 
@@ -78,4 +80,10 @@ app.use(`/api/v2/projects`, protectedRoute(), router.projectsRouter);
 app.use(`/api/v2/reports`, protectedRoute(), router.reportsRouter);
 app.use(`/api/v2/tools`, protectedRoute(), router.toolsRouter);
 
+// If a non-existent route is called. Must go after other routes.
+app.use('*', (_req, _res, next) => next(EndpointNotFound404));
+
+// Request error handler. Must go last.
+app.use(errorHandler);
+
 export default app;
diff --git a/express-api/src/middleware/errorHandler.ts b/express-api/src/middleware/errorHandler.ts
new file mode 100644
index 000000000..ad13a0ea7
--- /dev/null
+++ b/express-api/src/middleware/errorHandler.ts
@@ -0,0 +1,40 @@
+import { ErrorWithCode } from '@/utilities/customErrors/ErrorWithCode';
+import logger from '@/utilities/winstonLogger';
+import { NextFunction, Request, Response } from 'express';
+
+/**
+ * Handles errors and sends appropriate response.
+ * Use this as the last middleware in express.ts
+ *
+ * @param err - The error message or Error object.
+ * @param req - The Express request object.
+ * @param res - The Express response object.
+ * @param next - The Express next function.
+ */
+const errorHandler = (
+  err: string | Error | ErrorWithCode,
+  req: Request,
+  res: Response,
+  next: NextFunction,
+) => {
+  // Is this one of the valid input options?
+  if (!(typeof err === 'string' || err instanceof Error)) {
+    const message = `Unknown server error.`;
+    logger.error(message);
+    return res.status(500).send(message);
+  }
+  // Determine what message and status should be
+  const message = err instanceof Error ? err.message : err;
+  const code = err instanceof ErrorWithCode ? err.code : 500;
+  // Report through logger
+  if (code === 500) {
+    logger.error(message);
+  } else {
+    logger.warn(message);
+  }
+  // Return status and message
+  res.status(code).send(`Error: ${message}`);
+  next();
+};
+
+export default errorHandler;
diff --git a/express-api/src/server.ts b/express-api/src/server.ts
index 476df87b1..fa1e58a1f 100644
--- a/express-api/src/server.ts
+++ b/express-api/src/server.ts
@@ -2,19 +2,74 @@ import logger from '@/utilities/winstonLogger';
 import constants from '@/constants';
 import app from '@/express';
 import { AppDataSource } from '@/appDataSource';
+import { Application } from 'express';
+import { IncomingMessage, Server, ServerResponse } from 'http';
 
 const { API_PORT } = constants;
 
-app.listen(API_PORT, (err?: Error) => {
-  if (err) logger.error(err);
-  logger.info(`Server started on port ${API_PORT}.`);
-});
+/**
+ * Starts the application server and initializes the database connection.
+ *
+ * @param app - The Express application instance.
+ * @returns The server instance.
+ */
+const startApp = async (app: Application) => {
+  const server = app.listen(API_PORT, (err?: Error) => {
+    if (err) logger.error(err);
+    logger.info(`Server started on port ${API_PORT}.`);
+  });
+
+  // creating connection to database
+  await AppDataSource.initialize()
+    .then(() => {
+      logger.info('Database connection has been initialized');
+    })
+    .catch((err?: Error) => {
+      logger.error('Error during data source initialization. With error: ', err);
+    });
 
-// creating connection to database
-AppDataSource.initialize()
-  .then(() => {
-    logger.info('Database connection has been initialized');
-  })
-  .catch((err?: Error) => {
-    logger.error('Error during data source initialization. With error: ', err);
+  return server;
+};
+
+// Start the server here.
+// Set up in a why that the server could be restarted (reassigned) if needed.
+let server: Server<typeof IncomingMessage, typeof ServerResponse>;
+(async () => {
+  server = await startApp(app);
+})();
+
+/**
+ * Stops the application gracefully.
+ *
+ * @param exitCode - The exit code to be used when terminating the process.
+ * @param e - Optional. The error message to be logged.
+ *
+ * @returns void
+ */
+const stopApp = async (exitCode: number, e?: string) => {
+  logger.warn('Closing database connection.');
+  await AppDataSource.destroy();
+  logger.warn('Closing user connections.');
+  server.closeAllConnections();
+  server.close(() => {
+    logger.error(`Express application terminated. ${e ?? 'unknown'}`);
+    process.exit(exitCode);
   });
+};
+
+// Error catching listeners
+process.on('uncaughtException', (err: Error) => {
+  logger.warn('Uncaught exception received. Shutting down gracefully.');
+  stopApp(1, `Uncaught exception: ${err.stack}`);
+});
+
+process.on('unhandledRejection', (err: Error) => {
+  logger.warn('Unhandled rejection received. Shutting down gracefully.');
+  stopApp(1, `Unhandled rejection: ${err.stack}`);
+});
+
+// When termination call is received.
+process.on('SIGTERM', () => {
+  logger.warn('SIGTERM received. Shutting down gracefully.');
+  stopApp(0, 'SIGTERM request.');
+});
diff --git a/express-api/src/utilities/customErrors/ErrorWithCode.ts b/express-api/src/utilities/customErrors/ErrorWithCode.ts
index 2e84bd71c..707944385 100644
--- a/express-api/src/utilities/customErrors/ErrorWithCode.ts
+++ b/express-api/src/utilities/customErrors/ErrorWithCode.ts
@@ -1,7 +1,25 @@
+/**
+ * Represents an error with an associated error code.
+ *
+ * @class ErrorWithCode
+ * @extends Error
+ *
+ * @param {string} message - The error message.
+ * @param {number} code - The error code. Defaults to 400.
+ *
+ * @example
+ * const err = new ErrorWithCode('test');
+ * console.log(err.code); // 400
+ *
+ * @example
+ * const err = new ErrorWithCode('test', 401);
+ * console.log(err.code); // 401
+ * console.log(err.message); // 'test'
+ */
 export class ErrorWithCode extends Error {
   public code: number;
 
-  constructor(message: string, code?: number) {
+  constructor(message: string, code: number = 400) {
     super(message);
     this.code = code;
   }
diff --git a/express-api/tests/unit/middleware/errorHandler.test.ts b/express-api/tests/unit/middleware/errorHandler.test.ts
new file mode 100644
index 000000000..602f6d859
--- /dev/null
+++ b/express-api/tests/unit/middleware/errorHandler.test.ts
@@ -0,0 +1,39 @@
+import { Request, Response } from 'express';
+import errorHandler from '@/middleware/errorHandler';
+import { MockReq, MockRes, getRequestHandlerMocks } from 'tests/testUtils/factories';
+import { ErrorWithCode } from '@/utilities/customErrors/ErrorWithCode';
+
+let mockRequest: Request & MockReq, mockResponse: Response & MockRes;
+
+describe('UNIT - errorHandler middleware', () => {
+  beforeEach(() => {
+    const { mockReq, mockRes } = getRequestHandlerMocks();
+    mockRequest = mockReq;
+    mockResponse = mockRes;
+  });
+  const nextFunction = jest.fn();
+
+  it('should give error code 500 and send the passed error message when given a string', async () => {
+    errorHandler('string', mockRequest, mockResponse, nextFunction);
+    expect(mockResponse.status).toHaveBeenCalledWith(500);
+    expect(mockResponse.send).toHaveBeenCalledWith('Error: string');
+  });
+
+  it('should give code 500 and send the error message when given an Error', async () => {
+    errorHandler(new Error('error'), mockRequest, mockResponse, nextFunction);
+    expect(mockResponse.status).toHaveBeenCalledWith(500);
+    expect(mockResponse.send).toHaveBeenCalledWith('Error: error');
+  });
+
+  it('should give expected code and send the error message when given an ErrorWithCode', async () => {
+    errorHandler(new ErrorWithCode('withCode', 403), mockRequest, mockResponse, nextFunction);
+    expect(mockResponse.status).toHaveBeenCalledWith(403);
+    expect(mockResponse.send).toHaveBeenCalledWith('Error: withCode');
+  });
+
+  it('should give code 500 and generic error message when given something unexpected', async () => {
+    errorHandler(undefined, mockRequest, mockResponse, nextFunction);
+    expect(mockResponse.status).toHaveBeenCalledWith(500);
+    expect(mockResponse.send).toHaveBeenCalledWith('Unknown server error.');
+  });
+});
diff --git a/express-api/tests/unit/utilities/ErrorWithCode.test.ts b/express-api/tests/unit/utilities/ErrorWithCode.test.ts
new file mode 100644
index 000000000..8e39b8c88
--- /dev/null
+++ b/express-api/tests/unit/utilities/ErrorWithCode.test.ts
@@ -0,0 +1,13 @@
+import { ErrorWithCode } from '@/utilities/customErrors/ErrorWithCode';
+describe('UNIT - ErrorWithCode', () => {
+  it('should have a default 400 error code', () => {
+    const err = new ErrorWithCode('test');
+    expect(err.code).toEqual(400);
+  });
+
+  it('should use the error code provided', () => {
+    const err = new ErrorWithCode('test', 401);
+    expect(err.code).toEqual(401);
+    expect(err.message).toBe('test');
+  });
+});