The Common Gateway Interface (CGI) is one of the oldest tools for connecting web sites to program logic, and it’s still a common starting point. CGI provides a standard interface between the web server and applications, making it easier to write applications without having to build them directly into the server. Developers have been writing CGI scripts since the early days of the NCSA server, and Apache continues to support this popular and well-understood (if inefficient) mechanism for connecting HTTP requests to programs. While CGI scripts can be written in a variety of languages, the dominant language for CGI work has pretty much always been Perl. This chapter will explore CGI’s capabilities, explain its integration with Apache, and provide a demonstration in Perl.