This article draws on data and experiences observing and analyzing school lotteries from the National Evaluation of Charter School Impacts to describe challenges associated with lottery-based research. It presents evidence on the prevalence of oversubscribed charter middle schools and compares their features to charter schools that are not oversubscribed. It also describes the lotteries charter schools conduct and procedures for admitting students from waiting lists, as well as lessons for researchers for using lotteries to estimate impacts of schools on student outcomes.