This longitudinal study investigated the relative contributions of infant temperament, maternal sensitivity, and psychosocial risk to individual differences in preschool children's cognitive development. It also examined specific moderating effects between predictors as well as the specific mediating role of maternal sensitivity in the relation between psychosocial risk and children's cognitive development. A mixed sample comprising 27 low-risk (i.e., adult mothers) and 62 high-risk (i.e., adolescent mothers) mother-child dyads was evaluated at home on five occasions. At 6 and 10 months, infants' cognitive development was assessed using the Bayley Scales of Infant Development (BSID) (Bayley, 1993). At 15 and 18 months, mothers completed Goldsmith's (1996) Toddler Behavior Assessment Questionnaire (TBAQ), and observers completed Pederson and Moran's (1995) Maternal Behavior Q-Sort. At 36 months, preschoolers' cognitive development was reassessed using the BSID. Results showed that controlling for infant mental development scores, all three classes of variables contributed to differences in cognitive functioning. Psychosocial risk moderated the relations between interest persistence and preschooler cognitive development, and the interaction between risk and infant anger proneness tended toward significance (p = 0.07). Maternal sensitivity significantly, but partially, mediated the relation between psychosocial risk and cognitive development. Implications of these results for current understanding of the processes underlying socioemotional influences on cognitive development are discussed.