How does teacher change get conceptualized, supported, and realized? The authors address this far-reaching question by exploring how teacher collaboration, centered on examining student learning data, can result in changes in instructional perspectives and practices. Their case study consists of a middle school mathematics teacher group working with administrators and professional developers. A grounded theory approach is used to discuss their five-year journey, which includes two years prior to their formal beginning, and the various factors that impacted individual and group teacher change. They specifically address the following research question: What influences the long-term trajectory of a collaborative teacher inquiry team, and what is the nature of the changes that define this trajectory? (Contains 2 figures.)