The student population in contemporary public middle schools is growing increasingly more diverse. While heterogeneously grouped classrooms are consistent with middle school philosophy as well as recommendations from leading middle school advocacy groups, many teachers in these settings seem to downplay the differences among the students and "teach to the middle." This qualitative explores 200 middle school teachers' responses to an initiative in differentiation intended to address the variation in student academic readiness, interests, and learning profiles. Using systematic grounded theory methodology, a model of teacher change emerged including four categories of teacher responses to the differentiation initiative. This study's findings suggest that a variety of factors, including teachers' pre-existing beliefs about teaching and learning, and teachers' willingness and capacity for reflection greatly influence their responses to differentiation in heterogeneous, middle school classrooms. (Contains 1 table and 1 endnote.)