A 5-year longitudinal study of infant day care programs in New York City is described. Emphasis is on the effects of day care on the child and family, particulary with regard to mother-infant separation. The study is large scale (involving 550 children and families), and it focuses on service-oriented programs that have been developed in the past few years by many participating agencies. Three groups are being studied: (1) children in group day care centers, (2) children in family day care centers, and (3) home reared children. Data is largely based on systematic, naturalistic observations, although many tests of cognitive, emotional-social, and physical development are being used as outcome measures. Instruments to indicate effects on families are also being administered. Some cross-sectional analysis will be made, so that early versus late entry comparisons can be included. The project stresses the acceptance and cooperation of each of the participating agencies and parents involved; the steps taken to insure these attitudes are described. (DP)