The Ypsilanti Perry Preschool Project was designed to help economically disadvantaged children at high academic risk to cope more effectively with school and adult life in the mainstream of society. From 1962 through 1967, 123 children participated in the project. The basic experiment contrasted children who attended preschool with children who did not. Five pairs of experimental and control groups were used in five replications of the basic experiment from 1962 to 1967. This report extends the longitudinal study through fourth grade using data collected from all children. The independent variables of the study were the preschool program, the sex of the child, initial child measures (such as the Stanford Binet Intelligence Scale, and the Peabody Picture Vocabulary Test), and home environment variables. Dependent variables in the experiment consisted of four broad categories of measurement: academic aptitude and achievement, social and emotional development, home environment, and school success. Among the findings of the study are the following: (1) preschool attendance had an immediate and positive impact on the measured aptitude of children in the project; (2) while differences in measured aptitude gradually diminished during the post treatment period, differences in academic achievement between the experimental and control groups increased over time; (3) the overall pattern of differences in teacher ratings of children's social emotional maturity consistently and significantly favored the experimental group after kindergarten; and (4) experimental group children were found to be more successful in school as evaluated by the schools' own criteria of success. Appendices to this document describe measurement instruments and the statistical methodology of the study. (Author/AM)