Empirical evidence suggests that successful teacher professional development programs are intensive, ongoing, and connected to practice; focused on specific subject content; and foster strong working relationships among teachers. They support teacher motivation so that the acquired skills continue to be practiced in class. These critical elements are also embedded in collaborative learning structures. Research indicates that these collaborative contexts have an impact on teacher efficacy, an outcome that has been empirically linked to improved student achievement, and teacher adaptability and adjustment. This study used a qualitative lens to examine how lesson study provided the conditions identified in effective collaborative learning structures to support teacher efficacy. It was carried out in 3 subject domains with 10 teachers in a Singapore high school. It further explored the efficacy sources that facilitated the teachers’ collaborative efforts. The authors discuss implications for staff development programs in the light of these findings.