Interest in game-based learning for K-12 is growing. Thus, helping teachers understand how to use these new pedagogies is important. This paper presents a cross-case study of the development of teacher professional development for the River City project, a games-based multi-user virtual environment science curriculum project for middle school children, over three years of its development. Successful professional development required attention to multiple factors including teacher efficacy in using the software, pedagogical issues and school culture. A theoretical model for successful technological implementations is discussed. (Contains 1 table.)