While aggression is often conceived as a learned behavior that peaks during adolescence, this important volume shows that aggressive behaviors have their origins in early childhood and even infancy. Findings from major longitudinal research programs are used to illuminate the processes by which most children learn alternatives to physical aggression as they grow older, while a minority become increasingly violent. The developmental trajectories of proactive, reactive, and indirect aggression are reviewed, as are lessons learned from animal studies. Bringing together the best of current knowledge, the volume sheds new light on the interplay of biological factors, social and environmental influences, and sex differences in both adaptive and maladaptive aggression.