We evaluated the extent to which the externalizing behavior construct is self-organizing in the first 2 years of life. Based on dynamic systems theory, we hypothesized that changes in physical aggression, defiance, activity level, and distress to limitations would each be predicted by earlier manifestations of one another. These hypotheses were evaluated via mothers' and fathers' reports of 274 infants' externalizing behaviors at 8, 15, and 24 months of child age. Eight-month measures of physical aggression, activity level, and/or distress to limitations explained increases in physical aggression, defiance, activity level, and distress to limitations from 8 to 15 months. Increases in defiance and activity level from 15 to 24 months were predicted by 15-month physical aggression and/or distress to limitations. These findings suggest that the externalizing behavior construct is formed by dynamic interplay among its individual elements, particularly between 8 and 15 months.