This paper is a summary of results of two overlapping longitudinal studies tracing the development of mother-infant and father-infant attachments between 7 and 24 months of age. Interaction between 10 male and 10 female infants and their parents was observed in the subjects' homes. Interaction with parents was compared to interaction with an accessible and friendly investigator. Data concerning onset of attachment and presence of parental preferences were based on the occurrence of six attachment behaviors (proximity, touch, approach, reach, seek to be held, and fuss) and five affiliative behaviors (smile, vocalize, look, laugh and proffer). Frequency of parent vocalization to the child was recorded as an index of parental activity. Two classes of dyadic interaction were considered: play and physical contact. Results indicate that: (1) infants are attached to both parents from the time they are first able to form relationships (around 6-8 months of age); (2) mother-infant and father-infant relationships involve different types of experiences, and mothers and fathers have significant, qualitatively different influences on the infant's psychosocial development; and (3) the father-son relationship appears to be especially important from the beginning of the second year of life. Emphasis is placed on the need for attention to the particular nature of father-infant interaction. (Author/BF)