Head Start and other publicly funded preschool programs are some of the most popular government programs in the United States, and in recent years officials have explored expanding public preschool and making it universal. However, several recent largescale studies have raised questions about the benefits of these programs for participants and for society, as well as whether high-quality preschool is achievable on a large scale. This article reviews the available evidence on these questions and also what is known about the quality of various types of existing programs. The evidence indicates that the curriculum and professional development choices of most programs are out of step with the science of early childhood education and that this has made preschool programs less effective than they could be. The Boston Public Schools prekindergarten program can be used as a case study in better practice preschool implementation. Evaluation of this program shows that high-quality public preschool is achievable on a large scale if localities make the right investment and implementation decisions.