How do you implement middleware in FastAPI?

FastAPI (via Starlette) supports ASGI middleware. Built-in middleware: app.add_middleware(CORSMiddleware, ...), app.add_middleware(HTTPSRedirectMiddleware), app.add_middleware(TrustedHostMiddleware, allowed_hosts=["example.com"]). Custom middleware using Starlette's BaseHTTPMiddleware: class LoggingMiddleware(BaseHTTPMiddleware): async def dispatch(self, request, call_next): start = time.time(); response = await call_next(request); print(f"Request took {time.time()-start:.3f}s"); return response. Register: app.add_middleware(LoggingMiddleware). For pure ASGI middleware (lower level, more performant): wrap the app directly using the @app.middleware("http") decorator. Note: middleware wraps all requests including the auto-generated docs. Use dependency injection instead of middleware for route-specific concerns — it is more targeted and testable.