Robertson and Bowlby (1952) found that prolonged separations from the mother lower the extent to which infants seek proximity to their mother. Although prolonged separations are no longer common today, some infants experience extremely long hours of nonmaternal care, which may lead them to seek less proximity to their mother. I examined this hypothesis using data from the National Institute of Child Health and Development: Early Child Care and Youth Development Study (N = 1,281). A series of regression analyses revealed that infants’ hours of nonmaternal care at 4 to 6, 7 to 9, and 10 to 12 months, but not at 1 to 3 or 13 to 15 months, were associated with their proximity-seeking behavior in the Strange Situation at 15 months. Using a polynomial regression analysis, I further found a cubic relation between the number of nonmaternal care hours at 7 to 9 months and infants’ proximity-seeking behavior. Specifically, proximity-seeking behavior rapidly declined during two time periods: when infants spent from 0 to 10 hours per week in nonmaternal care and when they spent over 60 hours per week in nonmaternal care. I also found that mothers’ and nonmaternal caregivers’ sensitivity was associated with infants’ proximity-seeking behavior, and proximity-seeking behavior predicted young children's ability to control their behavior and also the amount of time that they were able to focus their attention on their mother or their experimenter during a developmentally challenging task at 36 months. Findings reported in this dissertation highlight the important role of proximity-seeking behavior in the attachment relationship formed with the caregiver during infancy and the development of self-control and attention during the preschool years. / text