Children who experience early and extensive child care, especially center-based care, are rated by teachers as having more externalizing behavior problems than are other children. This association is reduced, but not eliminated, when care is of high quality, and it varies by socioeconomic disadvantage and the type of behavior assessed. We examine the processes that may account for the quantity effect, concluding that it occurs primarily among relatively advantaged White non-Hispanic families. It appears primarily for teacher-rated behavior, especially externalizing and low self-control, but is not evident for positive behavior and peer interaction skills. Some of the processes accounting for the relation of quantity to behavior are most likely to be poor caregiver-child relationships and negative peer interactions, not reduced attachment to mothers or lowered maternal sensitivity. Many questions remain about duration of effects, developmental and individual differences, more nuanced conceptualizations of both care quality and social behavior, and variations across cultural and ethnic groups.