This study examined the impact of autonomic self-regulation, affective experience, and effortful control on externalizing behavior in preadolescents (n = 360) deemed at-risk for aggression. Autonomic self-regulation was assessed using measurements of respiratory sinus arrhythmia and skin conductance levels as markers for parasympathetic and sympathetic functioning. Affective experience and effortful control constructs were derived using scales from the Early Adolescent Temperament Questionnaire. Structural equation modeling indicated that sympathetic and parasympathetic functioning predicted externalizing behavior even when accounting for the influence of temperament traits. These significant associations were small in magnitude. As expected, higher parasympathetic and lower sympathetic reactivity predicted greater externalizing behavior. Effortful control and affective experience also independently predicted problem behavior when accounting for autonomic functioning and each other’s influence as evidenced by small to medium significant associations. Lower levels of these constructs predicted higher problem behavior. Implications for the prevention and treatment of externalizing behavior are discussed.