Effective teachers make a real difference for student learning. But research shows that both in Massachusetts and nationwide, academically struggling students and those from historically low performing subgroups are less likely to be assigned to the teachers who are most likely to generate strong results. This results in missed opportunities to close achievement gaps and increase educational outcomes for all students. This policy brief provides an overview of how effective teachers are identified, summarizes research from around the nation, and analyzes Massachusetts data to address several important questions. It also provides connections to resources available to Massachusetts schools and districts working to eliminate equity gaps.