In the following sections, we summarize what research shows about preschool education's effectiveness across a number of developmental domains. We look at effects across three time frames: immediately after preschool, during elementary and middle school, and during adolescence and adulthood. We describe how effects vary according to how intensive the preschool programs are and how long they last, family and child characteristics, and program quality. Finally, we highlight the strongest findings and discuss what we still need to know to help both policy makers and educators. (author abstract)