More than one in five American children lives in poverty. By the time they start kindergarten, these children are already behind their middle-class peers in reading and math, and that gap only widens as they continue in school. More than 40 years ago, though, seminal studies showed that comprehensive and high-quality preschool programs could have tremendous, lasting effects on these children’s lives. In the last 15 years, the movement has picked up steam, with more and more states and localities expanding access to preschool. But many important questions remain about how to make the most of the promise of preschool.