Educational computer games provide an environment in which interactions among students, teachers, and texts differ non-trivially from those of the traditional classroom. In order to build and research computer games effectively, it is important to provide a theoretical background that adequately describes and explains learning and interactions in a virtual environment. This article presents the theory of communities of practice as one possible theoretical backing. Five aspects of this theory are presented: shared goals, shared practices, legitimate peripheral participation, co-evolution and adaptation, and identity development. Each aspect is explained and related to educational computer games. Suggestions for game designers and researchers are also given.