Over the past decade, charter schools have gained a prominent role in the debate over how to improve K-12 education, becoming one of those rare ideas that attracts bipartisan support. Yet as charter schools have expanded and begun educating more American school children, so has discussion over the role charter schools should ultimately play in our public school system. This policy brief provides an overview of what charter schools are (and are not) and what we know about their effectiveness, with the aim of facilitating more knowledgeable discussion among policymakers and other stakeholders.