This study compares the reading and mathematics proficiency of charter school students to that of their fellow students in neighboring public schools. Ninety-nine percent of all elementary students in charter schools are included in the study. The charter schools are compared to the schools that their students would most likely otherwise attend: the nearest regular public school with a similar racial composition (the "matched" school). Compared to students in the matched regular public school, charter students are 5.2 percent more likely to be proficient in reading and 3.2 percent more likely to be proficient in math on their state's exams. Students in charter schools that have been in operation longer are more likely to have a proficiency advantage over their peers in the matched regular public school. In reading, the advantage is 2.5 percent for a charter school that has been operating 1 to 4 years, 5.2 percent for a school operating 5 to 8 years, and 10.1 percent for a school operating 9 to 11 years. Also, charter school students are more likely to have a proficiency advantage if their school has funding that is at least forty percent of that enjoyed by regular public schools. The results suggest that charter schools are especially likely to raise the achievement of students who are poor or Hispanic.