Deficits in executive function, and in particular, reduced capacity to inhibit a dominant action, are a risk factor for externalizing problems (EP). Inhibitory control (IC) develops in the later preschool and early childhood periods, such that IC might not regulate EP in toddlers and younger preschoolers. Aggression was observed during peer play for 66 girls and 49 boys, from 2.75 to 6.00 years (M = 4.14, S.D. = 0.78). Mothers reported on children's IC and EP concurrently and 12 months later, and concurrent teacher reports of EP were also collected. Factor analysis supported aggregation of mother and teacher-reported EP and observed physical aggression into one measure of externalizing difficulties. Mothers reported lower IC for children with more externalizing difficulties, and the inverse relation between IC and externalizing difficulties strengthened over the toddler, preschool and kindergarten periods. Similar relations between IC and EP were observed 12 months later, and increases in IC also predicted reductions in EP over 1 year. These data demonstrate that the preschool years are a dynamic period of developmental change in the relations between IC and EP. (Contains 1 note, 3 tables, and 2 figures.)