It began as a program evaluation but now, half a century later, serves as a test of the long-term effects and return on investment of high-quality preschool education for young children living in poverty. This study was conducted in the United States, beginning in the 1960s, and has generated data on study participants from birth through 40, with new data now being collected at age 50. The study used random assignment procedures to assign 123 children to a preschool program and a control group who receive no preschool program. Program participants surpassed non-participants in intellectual performance at school entry, school achievement throughout schooling, commitment to schooling, high school graduation rate, adult employment rate and earnings, reduced childhood antisocial behavior, and reduced adult crime and incarceration. The program's return on investment was at least seven times as great as its operating cost. While these powerful results have been found not only in this study but in several similar studies, they have not been found in studies of larger preschool programs, such as the Head Start Impact Study. This discrepancy suggests that differences between the two types of programs account for the better results found in studies such as this one. Among these differences are highly qualified teachers, a valid child development curriculum, extensive engagement of parents, and regular assessment of program implementation and children's development. Abstract published by arrangement with Springer.