Charter schools, first launched in the 1990s, are an important and growing component of the public school system in the United States. More than 5,000 charter schools serve over 1.5 million students (approximately 3% of all public school students, in 40 states). There remains considerable debate as to whether, how, and under what circumstances charter schools improve the outcomes of students who attend them. This book summarises the results of a new study: the Evaluation of Charter School Impacts, a large-scale randomised trial of the effectiveness of charter schools funded by the Institute of Education Sciences and conducted by Mathematica Policy Research and its partners.