This policy brief reviews research on what impact the competition introduced by vouchers and charter schools has upon the effectiveness and efficiency of traditional public schools. Only recently has such research been possible in the United States, as choice options became sufficiently widespread to elicit competitive responses from traditional public schools. The authors summarize conflicting theoretical predictions about how competition affects students who do not actively choose, and they identify features of policy design, implementation and local settings likely to influence the nature of competition. They find that results from available empirical studies are mixed and do not yet allow for firm conclusions about the effects of competition on traditional schools and non-choosing students. The review notes methodological challenges and possible lines of future research.