Teachers in a middle school and a high school with high percentages of English language learners showed evidence of transformative learning during the 2 years in which they collaborated in an ongoing professional development activity called "learning labs," which include focused discussion of observations of colleagues' classrooms. An analysis of qualitative data found teachers' stated beliefs about their own and students' roles, responsibilities, and capabilities changed as a result of their participation in learning labs. Teachers also implemented changes in their instructional practices. The authors argue that effective professional development activities must go beyond the delivery of information to support teachers' examination and revision of assumptions about students, teaching, and learning that guide their practice. (Contains 3 tables, 1 figure, and 1 footnote.)