This study examined physiological correlates and predictors of relational and physical aggression in early childhood. Preschoolers’ baseline heart rate, blood pressure, and respiratory sinus arrhythmia (RSA) were measured in the fall. At the same assessment, heart rate and RSA reactivity were measured while listening to stories of peer conflict, and participants engaged in two effortful control tasks. Teachers reported on physical and relational aggression in the fall and the spring. With respect to baseline physiology, low baseline heart rate and higher RSA were associated with increased physical aggression only among children with lower effortful control scores. Higher baseline RSA predicted increased relational aggression, again only for children with lower effortful control scores. Among children with poorer effortful control, diastolic blood pressure positively predicted relational aggression and negatively predicted physical aggression. Greater heart rate increases and RSA decreases to stories of peer conflict were uniquely associated with elevated classroom physical aggression. These findings suggest the utility of examining the roles of baseline physiology and physiological reactivity in the development of aggressive behavior. Implications of these findings for the development of intervention and prevention programs targeting early physical and relational aggression are discussed.