The charter-school sector in the United States has grown steadily since the first charter school opened in 1992 and nearly 7,000 charter schools currently serve nearly 3 million students. Research suggests that the average charter school performs about the same as nearby traditional public schools, but there is great variation in the effects of charter schools. Some are successful in boosting student achievement and others are not, raising the question of what distinguishes good charter schools from bad. This article summarizes the research on factors associated with successful charter schools. The research suggests that urban charter schools have more positive impacts on student achievement than other charter schools. The policies most consistently found to be associated with positive charter-school impacts include long school days or years, comprehensive behavioral policies with rewards and sanctions, and a mission that prioritizes boosting student achievement. In addition, moderately strong evidence suggests that high-dosage tutoring, frequent feedback and coaching for teachers, and policies promoting the use of data to guide teachers' instructional practices are positively associated with charter schools' achievement impacts.