Next.js middleware provides developers with a powerful tool to customize request handling in their applications, allowing for operations such as session validation, logging, and caching. Middleware functions run automatically for every incoming request, enabling inspection or modification of request data before it reaches the routing system. While offering flexibility and customizability, middleware must be used judiciously to avoid performance issues. Key use cases include authentication, logging, data fetching, request routing, caching, rate limiting, page transformations, analytics, and internationalization. Middleware operates within the Edge Runtime, which imposes constraints on APIs and libraries, limiting access to request and response objects and preventing the use of native Node.js APIs or large code bundles. Despite these limitations, middleware enhances the robustness, scalability, and maintainability of Next.js applications, though developers must ensure their middleware code is lightweight and efficient to maintain optimal application performance.