An evaluation of the New York State Experimental Prekindergarten Program was begun in 1975 to provide longititudinal data on the effects of prekindergarten on children's development over a period of 5 years. Data were collected on a variety of items, including children's test performance at various times, family background, ratings by teachers, attendance, services received, teachers' observations and other documentary materials, parents' participation in the program, and children's progress through school. Measures of performance included three areas of cognitive functioning: general reasoning, school-related knowledge and skills, and knowledge of verbal concepts. Noncognitive and behavioral characteristics such as task orientation, extraversion, and verbal facility were also assessed. In addition to the study of program outcomes, an attempt was made to increase the degree of program continuity in seven districts and to identify lasting effects of continuity on learning. The children of greatest interest were those who entered prekindergarten in the fall of 1975. To corroborate results obtained for the first group tested, 4 years of data were collected on a second group entering prekindergarten in the fall of 1976. Pretests and posttests were administered to a sample of children from each class entering in 1977 through 1979 and, for the first two groups, two control segments were established. The development of analytical procedures is described, and results are reported and discussed. (RH)