The Common Gateway Interface (CGI) specification can be found at the original NCSA site < http://hoohoo.ncsa.uiuc.edu/cgi/interface.html>. This version hasn't been updated since 1995, and there have been some efforts to update it.

A new draft is being worked on with the intent of making it an informational RFC; you can find out more about this project at <http://web.golux.com/coar/cgi/>.