Prior research has documented associations between hours in child care and children's externalizing behavior. A series of longitudinal analyses were conducted to address 5 propositions, each testing the hypothesis that child care hours causes externalizing behavior. Data from the National Institute of Child Health and Human Development Early Child Care Research Network (NICHD) Study of Early Child Care and Youth Development were used in this investigation because they include repeated measures of child care experiences, externalizing behavior, and family characteristics. There were 3 main findings. First, the evidence linking child care hours with externalizing behavior was equivocal in that results varied across model specifications. Second, the association between child care hours and externalizing behavior was not due to a child effect. Third, child care quality and proportion of time spent with a large group of peers moderated the effects of child care hours on externalizing behavior. The number of hours spent in child care was more strongly related to externalizing behavior when children were in low-quality child care and when children spent a greater proportion of time with a large group of peers. The magnitude of associations between child care hours and externalizing behavior was modest. Implications are that parents and policymakers must take into account that externalizing behavior is predicted from a constellation of variables in multiple contexts. (Contains 7 tables and 3 figures.)