Externalizing behaviors observed in early childhood have been found to be stable, particularly for boys, but little research has investigated the antecedents of these behaviors, especially how the antecedents may differentially relate to externalizing behaviors in boys and girls. The goal of this study was to explore predictors of externalizing behaviors concurrently in toddlerhood and longitudinally to preschool. When children (n = 140) were 30-36 months old, maternal supportive and controlling behaviors were observed, and children's effortful control and anger were measured through observations and maternal report. Mothers reported on children's externalizing behavior during toddlerhood and approximately 2 years later (n = 116). Although mean level differences were not found between boys and girls, effortful control was differentially related to externalizing behaviors in toddlerhood. Higher levels of effortful control were associated with less externalizing behaviors for boys but not for girls. Additionally, anger was positively related to externalizing behaviors. Few associations were found for maternal behaviors, which emphasizes the importance of child characteristics in externalizing behaviors. Our findings emphasize how future research should continue to examine relations of early antecedents to concurrent and later externalizing behaviors even if mean level sex differences are not found.