There is a broad consensus that the United States should expand its current preschool programs, particularly for disadvantaged students. This consensus reflects both a general interest in improving the preparation of students entering schools and a particular concern that disadvantaged students are especially handicapped by current preschool educational experiences. Matched with this desire to improve school preparation is evidence that at least some preschools are able to significantly improve the outcomes of their students. When considering a mass expansion of public pre-K, two central questions arise. First, what should be the characteristics of new, broader preschool programs? Second, how best should such programs be taken to scale? Unfortunately, available evidence provides limited guidance on both issues. Because we lack basic information about program design and program implementation, it would be foolhardy to move directly into a broad universal pre-K program. We should act on preschool, but in a way that generates information on how to develop and improve programs.