Background/Context: Many urban schools today look to instructional teams as a means to decrease professional isolation, promote teachers' ongoing development, and substantially reduce well-documented variation in teachers' effectiveness across classrooms. Recent research finds that teams can contribute to teachers' development and increased student achievement. However, research also suggests that teams often fail and that most schools are not organized to ensure their success. Therefore, it is important to learn more about how teams function in successful schools, how teachers experience them, and what factors contribute to their success. Purpose/Objective/Research Question/Focus of Study: Data for this article were drawn from a comparative case study focusing on the human-capital practices in six successful high-poverty, high-minority schools (traditional, turnaround, restart, and charter), all located in one Massachusetts city. Each school was affected by a distinct set of state and local policies. Here, we focus on the schools' approaches to professional learning and collaboration among teachers. Did they rely on teams, and, if so, what purposes did the teams serve, and how were they organized? How did teachers assess their experience with teams? What role did administrators play? Were there notable school-to-school differences in how these teams were organized and managed? Research Design/Data Collection and Analysis: For this qualitative, comparative case study, we conducted semistructured interviews with 142 teachers, administrators, and other staff in six elementary and middle schools. Interview protocols encouraged participants to discuss their school's approach to teachers' professional learning and work with colleagues. During school visits, we also observed a wide range of day-to-day practices and collected documents describing school policies and practices. We coded our data with both emic and etic topical codes and used various matrices to analyze responses within and across the sites. Conclusions/Recommendations: Five schools relied on teams as a central mechanism for school improvement, dedicating substantial blocks of time each week to teachers' meetings. Teams focused on matters of content (curriculum, lesson plans, and student achievement) and the student cohort (individual progress, group behavior, and organizational culture). Teachers valued their work on teams, saying that it supported their instruction and contributed to their school's success by creating coherence across classrooms and shared responsibility for students. Factors that supported teams included having a worthy purpose in support of the school's mission; sufficient, regular time for meetings; engaged support by administrators; and facilitation by trained teacher-leaders.