This article examines the relationship between teachers' unions and teacher turnover in U.S. public schools. The trade-off between teacher pay and employment predicts that unions raise the dismissal rate of underperforming teachers but reduce the attrition of high-quality teachers, as the higher wages unions negotiate provide districts strong incentives to scrutinize teacher performance during a probationary period while encouraging high-quality teachers to remain in teaching. Using the district-teacher matched data and a natural experiment, I find that, compared to less-unionized districts, highly unionized districts dismiss more low-quality teachers and retain more high-quality teachers, raising average teacher quality and educational outcomes.Recent studies suggest that teachers are the most important school factor in improving the educational achievement of U.S. students (Burke and Sass 2006; Goldhaber, Brewer, and Anderson 1999; Ingersoll and Merrill 2017;Rivkin, Hanushek, and Kain 2005). Yet, researchers find that the quality of the teaching workforce in the United States has been declining for the past several decades (Bacolod 2003;Corcoran, Evans, and Schwab 2004; Hoxby and Leigh 2004;Lakdawalla 2001; Murnane et al. 1991). Raising teacher quality, therefore, becomes a primary objective for policymakers in educational reforms. Staffing classrooms with high-quality teachers can be achieved by both hiring and retaining good teachers, and teacher turnover is a key factor linking both approaches.