The early emergence and developmental implications of externalizing behavior problems intoddlers and preschoolers are discussed with an emphasis on which young children are truly atrisk for continuing problems. The extant literature is reviewed with a focus on the stability ofearly externalizing behavior and the diverse pathways that young children, primarily boys, withearly-emerging problems may follow. Findings from a number of studies, both epidemiologicaland high risk, suggest that the small subgroup of boys with multiple risk factors that includeespecially high levels of early hyperactivity and aggression, and high levels of negative parentingand family stress, are most likely to evidence continuing problems at school entry.Sociodemographic and neighborhood influences are also discussed, as are implications for futureresearch and policy.