John Hattie has demonstrated in repeated meta-analyses that collective efficacy is the number one impact on student achievement (2017). As schools build an understanding about the research around collective efficacy and why it is essential, the next question is, how do we build it? Supervisory systems of teacher evaluation don’t build collective efficacy — in fact, they may actually create divisiveness (Popham, 2013). As we rethink appraisal to focus on professional growth, one of the best approaches to building collective efficacy is to create opportunities for teachers to see each other teach. This has long been a challenging issue in schools as teachers resist what they perceive as peer observation and evaluation. Done well, however, collegial work in each others’ classrooms fosters exactly the collaborative efforts that build collective efficacy.