This intensive longitudinal study of mother-infant interaction during the first year of life focuses on the development of attachment. Data on 26 middle-class families were collected by five methods: (1) naturalistic observation of each mother-infant pair during 4-hour home visits, which occurred at 3-week intervals from the infants' 3rd to 54th week of life; (2) observation in a controlled laboratory situation (the "strange situation"), beginning when infants were 51 weeks of age; (3) non-directive interviews with the mother and other adults in the household; (4) testing with the Griffiths' Infant Intelligence Scale approximately every nine weeks; and (5) examination of medical records. Observational data were emphasized in the data analysis. The analysis is discussed extensively under four headings: normative records of attachment behaviors, individual differences, hierarchical organization of attachment behavior, and the influence of environmental variation, specifically maternal behavior. (GO)