The FastCGI Extension for IIS 6.0 enables popular application frameworks that support FastCGI protocol to be hosted on the IIS web server in a high-performance and reliable way. FastCGI provides a high-performance alternative to the Common Gateway Interface (CGI), a standard way of interfacing external applications with Web servers that has been supported as part of the IIS feature-set since the very first release.

CGI programs are executables launched by the web server for each request in order to process the request and generate dynamic responses that are sent back to the client. Because many of these frameworks do not support multi-threaded execution, CGI enables them to execute reliably on IIS by executing exactly one request per process. Unfortunately, it provides poor performance due to the high cost of starting and shutting down a process for each request.

FastCGI addresses the performance issues inherent to CGI by providing a mechanism to reuse a single process over and over again for many requests. Additionally, FastCGI maintains compatibility with non-thread-safe libraries by providing a pool of reusable processes and ensuring that each process will only handle one request at a time.