This paper estimates the effect of charter schools on both students attending them and students at neighboring public schools. Using school-level data from Michigan's standardized testing program, I compare changes in test scores between charter and public school students. I find that test scores of charter school students do not improve, and may actually decline, relative to those of public school students. The paper also exploits exogenous variation created by Michigan's charter law to identify the effects of charter schools on public schools. The results suggest that charter schools have had no significant effect on test scores in neighboring public schools.