World of Goo was written in a highly data-driven manner, consequently almost every aspect of the game is customisable through data files (generally XML). The pages in this book document the various file formats and show how to create custom addins.

The goomod file format was developed for authors to distribute their addins for easy installation by end-users. It is not supported by the game itself, but is used by GooTool to customise the game.

See also the addin development FAQ.