This article is an introduction to mod files: their uses, differences, and importance both to using and making mods.

Plugins

Master Files(.esm) and Plugin Files(.esp) are the core data files of Fallout. By tradition, the file extensions stand for Elder Scrolls Master and Elder Scrolls Plugin. A master file acts as a database of all of the data for the world, including object data, dialogue, gameplay settings, object placements, AI settings, landscape, cells, etc. The Creation Kit is the tool we use to view and edit master files and plugins. Master files and plugins are largely identical in format, but have some important distinctions in practice. A master file is autonomous; it relies on no information other than itself. The Creation Kit will not create master files. A plug-in (ESP) file relies on information from a Master ESM file. Plug-ins CANNOT refer to information in another plug-in. They can only refer to data from a master file (though they can refer to multiple master files).

Archives

• Note that not all game data is contained in the master and plugin files. Textures, meshes, sounds, videos, etc. are all part of the full game, and are stored separately in Archives (.bsa files) or as loose files.