Introduction: The purpose of this study is to provide a framework for analysis from which to interpret the transformations that take place, as perceived by the participants, when commercial video games are used in the classroom. We will show how Activity Theory (AT) is able to explain and interpret these changes. Method: Case studies are presented, using a qualitative, ethnographic approach. These studies explore the transformations that take place in a secondary school resource room, when we introduce the Sim3 video game. Participating in the studies were the special education teacher and two groups of students with special educational needs (12-16 years old). Results: Video games, as mediational instruments, introduce practices, codes and languages into the classroom that provoke tension between elements of the system (the object of the activity, the relationship and role of community participants, outcomes of the activity and the rules of organization). Overcoming these tensions has involved transforming the resource room into a new, more facilitative system in order to respond to the special educational needs of students. Discussion: This study presents a framework for analysis, based on Activity Theory, that provides certain keys to interpreting the scholastic role of technological instruments, such as the video game: the importance of looking outside the classroom to other systems to which it is connected; viewing the video game as a cultural object that introduces new practices which come into conflict with the practices of teachers and students in a traditional classroom; and finally, focusing the analysis on the many perspectives and "voices" of the participants and on how their interpretations, beliefs and conceptions, are fundamental to change.