In this chapter, we present evidence about the effects of a school-based early-childhood intervention on children's well-being in the short and long term using data from the Chicago Longitudinal Study. The federally funded Chicago Child-Parent Centers provide education and family support services to low-income children from preschool to third grade. The centers are the nation's second oldest federally funded preschool program. Since 1967 more than 100,000 families have been served. Program effects are summarized up to age 22 for preschool, school-age, and extended-intervention components. Findings are summarized about the program's cost-effectiveness, the first such analysis for a public early-childhood program. Finally, the personal, family, and school factors that mediate the effects of program participation on long-term outcomes are identified. This is followed by a section on implications for policy and program development. (author abstract)