APIs are the backbone of modern applications, and building efficient, secure APIs is crucial for any web or mobile app. In 2024, best practices for API development with Express.js involve structuring routes in a clean, maintainable way, using middleware to handle requests, and ensuring proper authentication and authorization with JWTs or OAuth. Error handling, rate limiting, and using caching mechanisms like Redis can significantly improve performance and user experience. It's also important to document APIs using tools like Swagger or Postman to ensure developers can easily interact with them. Following RESTful principles and, when necessary, considering GraphQL for more complex data querying needs can make your API robust and scalable.