Early childhood interventions can be highly cost effective when positive impacts are sustained into adulthood. Yet while many recent preschool interventions have been found to have short-term effects on young children's language, literacy, mathematics, executive function, and social-emotional development, studies show that impacts on cognitive and academic skills tend to diminish in early elementary school--a phenomenon commonly known as fade-out or convergence. There are a number of plausible hypotheses, but little hard evidence, on how to sustain the benefits of early childhood education. This brief introduces the ExCEL P-3 project, a study being done in partnership with the Boston Public Schools, the University of Michigan, and the Harvard Graduate School of Education, which aims to explore several leading approaches for sustaining children's early preschool gains. Two related ExCEL projects--focusing on instructional quality (ExCEL Quality) and summer enrichment programs (ExCEL Summer)--will be covered in later briefs in this series.