Associations between maternal attachment patterns and cognitive skills at first grade were examined, and mediators of identified associations tested using Phase I and II data from the National Institute of Child Health and Human Development Study of Early Care and Youth Development, a prospective study of 1364 children and families from birth through sixth grade. There were three main findings. First, ambivalent and insecure/other attachment patterns were negative predictors of cognitive skills. Second, the association between ambivalent attachment and cognitive skills was mediated by children's behaviors related to testing. Third, the association between insecure/other attachment and cognitive skills was mediated by children's exploration, maternal instruction, children's social relationships, and children's behaviors related to testing. Implications for attachment theory and education are discussed.