We use student assignment lotteries to estimate the effect of charter school attendance onstudent achievement in Boston. We also evaluate a related alternative, Boston's pilot schools. Pilot schools have some of the independence of charter schools but are in the Boston Public School district and are covered by some collective bargaining provisions. Lottery estimates show large and significant score gains for charter students in middle and high school. In contrast, lottery estimates for pilot school students aremostly small and insignificant, with some significant negative effects. Charter schools with binding assignment lotteries appear togenerate larger gains thanother charters. © The Author(s) 2011. Published by Oxford University Press, on behalf of President and Fellows of Harvard College. All rights reserved.