Fiddler is an HTTP debugging proxy server application written by Eric Lawrence, formerly a Program Manager on the Internet Explorer development team at Microsoft.

Fiddler captures HTTP and HTTPS traffic and logs it for the user to review (the latter by implementing man-in-the-middle interception using self-signed certificates).

Fiddler can also be used to modify (“fiddle with”) HTTP traffic for troubleshooting purposes as it is being sent or received.  By default, traffic from Microsoft’s WinINET HTTP(S) stack is automatically directed to the proxy at runtime, but any browser or Web application (and most mobile devices) can be configured to route its traffic through Fiddler.