Scrapy middleware is a powerful mechanism divided into two types: downloader middleware and spider middleware. Downloader middleware is located between the engine and the downloader, allowing processing of requests and responses before they are sent to the downloader and after they are returned from the downloader. Common uses of downloader middleware include adding or modifying request headers, handling cookies, setting proxies, handling redirects, processing exceptions, and modifying response content. Spider middleware is located between the engine and the spider, allowing processing before responses are passed to the spider and after requests are returned from the spider. Common uses of spider middleware include filtering duplicate requests, modifying requests or responses, and handling spider exceptions. Middleware implements custom logic through methods such as process_request, process_response, and process_exception. Developers can enable or disable middleware through configuration files and set middleware priorities to control execution order. The use of middleware gives Scrapy high flexibility and scalability.