Over the past century, countries around the globe have empowered constitutional courts to safeguard the rule of law. But when can courts effectively perform this vital task? Drawing upon a series of survey experiments fielded in the United States, Germany, Hungary, and Poland, this book demonstrates that judicial independence is critical for judicial efficacy. Independent courts can empower citizens to punish executives who flout the bounds of constitutional rule; weak courts are unable to generate public costs for transgressing the law. Although judicial efficacy is neither universal nor automatic, courts - so long as they are viewed by the public as independent - can provide an effective check on executives and promote the rule of law.