The influence of early relational experience on later social understanding has evoked rich theoretical discussion but relatively little empirical inquiry. Enlisting data from the NICHD Study of Early Child Care and Youth Development, measures of the security of attachment in infancy, toddlerhood, and early childhood, together with measures of parenting quality (maternal sensitivity and depressive symptoms) gathered longitudinally throughout infancy and early childhood, were used to predict differences in children's thoughts and feelings about peers (i.e., social problem solving, negative attributional biases, aggressive solutions to ambiguous social situations, and self-reported loneliness) when children were 54 months and in first grade. Relational experiences, especially before 36 months, were significantly predictive of later peer-related representations. Attachment security at 24 and 36 months was associated with enhanced social problem-solving skills and less loneliness, but security of attachment at 15 months was nonpredictive. Early maternal sensitivity was positively associated with later social problem-solving and negatively with aggressive responses, and early maternal depressive symptoms were positively associated with children's negative attributions. Concurrent parenting quality was also associated with children's thoughts and feelings about peers, but less consistently. These findings shed new light on how early relational experiences may contribute to social information processing with peers at the end of the preschool years, and that the timing of relational influences may be crucial.