Since the turn of the 21st century, collective teacher efficacy has been positively associated with improved student outcomes, even after controlling for students' socioeconomic circumstances or prior achievement. Despite a large body of literature examining professional learning for teachers, little attention has been paid to intentionally fostering teachers' collective efficacy. Bandura posited 4 sources of information that contribute to the formation of efficacy beliefs, but limited research explicitly links the sources to processes and structures of professional learning. In this article, we offer insights into how collective teacher efficacy may have been shaped in the context of an Australian primary mathematics professional learning program. Through detailed descriptions of the program's structures and processes aligned with Bandura's 4 sources, we consider their potential to inform collective teacher efficacy. We conclude with recommendations for practice and further research.