In 2015, 42 states and the District of Columbia spent $6.2 billion in state funds on pre-Kindergarten (pre-K) programs, a fact that represents a growing commitment to pre-K as a way to help children from disadvantaged families increase their school readiness. But while numerous studies have documented the success of pre-K programs in preparing students for elementary school, inconclusive evidence about the sustainability of pre-K benefits as children move through their school years is raising important new questions for scientists, educators, and policymakers alike. How can states optimize their pre-K programs to provide both the strongest early learning boost and a solid foundation for future learning?