What is the value added of a year of preschool education? In an effort to answer the question, Abt Associates supported the Harvard Graduate School of Education in evaluating the efficacy of two early childhood education programs implemented in the Boston Public Schools: Opening the World of Learning (OWL) and Building Blocks for Math and Science. The research, conducted for the U.S. Department of Education, used a regression discontinuity design. Children in the treatment and control groups were tested for achievement in math and literacy, self-regulation and executive function, and social skills during the beginning of the 2009 - 2010 school year. Previous research on the added value of a year of preschool education had not been conclusive. This study, using a more rigorous evaluation approach, provides stronger evidence to help policymakers and educators make decisions regarding the place of preschool education in the instruction of children.