Teacher collaboration continues to be a pervasive strategy for capacity building and promoting student learning. While numerous studies provide insights into the characteristics and outcomes of teacher collaboration, prior research minimally addresses how teachers’ collaboration experiences evolve over time. However, such knowledge has the potential to shape how effective collaboration is developed and sustained, particularly in schools focused on capacity building. This multiple case study explores how teachers experience collaboration in urban schools under pressure to improve student achievement over four years. Data were collected through a research-practice partnership focused on school improvement through math teacher collaboration and coaching. I draw on 165 interviews with teachers, school leaders, and an instructional coach, as well as over 200 hours of observations of collaboration meetings and full-day capacity-building workshops. Findings are presented in three articles focused on distinct aspects of collaboration over time, including the content of collaborative conversations, the role of leaders in framing collaboration, and the intersection between teachers’ emotions and collaboration. Findings indicate that collaboration is a valuable lever for capacity-building, but requires careful cultivation to be effective. For some of the teams studied, collaboration provided high-depth teacher learning opportunities and fostered meaningful instructional improvement efforts. However, these successes were not routine and strongly influenced by shifts in contextual conditions. Factors influencing collective work include personnel transitions, accountability pressures, and tools mediating collaborative conversations. By foregrounding the role of contextual factors shaping teachers’ collective capacity building efforts over the course of four years, this dissertation study provides novel and actionable insights into fostering effective collaboration.