Collective efficacy refers to the shared perceptions of educators that, through their combined efforts, they can "organize and execute the courses of action required to have a positive effect on students" (Goddard, Hoy, & Hoy, 2004, p. 4). Tschannen-Moran and Barr (2004) expand on this definition, describing collective efficacy as the "collective self-perception that teachers in a given school make an educational difference to their students over and above the educational impact of their homes and communities" (p. 190). In fact, collective efficacy is what matters most in improving student learning, topping Hattie's (2016) list of factors that influence student achievement in schools. Many school leaders are asking: How do we foster a sense of collective efficacy among teachers to realize better outcomes for students? To answer that question, this article looks at why collective efficacy is important, how collaborative teacher inquiry can enhance collective efficacy, and how school leaders can support educators' engagement in the process.