This chapter is for developers or those who would like to extend the functionality of relax. It is not required for using relax. If you would like to make modifications to the relax source code please subscribe to all the relax mailing lists (see the open source infrastructure chapter for more details). Announcements are sent to ``relax-announce at gna.org'' whereas ``relax-users at gna.org'' is the list where discussions about the usage of relax should be posted. ``relax-devel at gna.org'' is where all discussions about the development of relax including feature requests, program design, or any other discussions relating to relax's structure or code should be posted. Finally, ``relax-commits at gna.org'' is where all changes to relax's code and documentation, as well as changes to the web pages, are automatically sent to. Anyone interested in joining the project should subscribe to all four lists.