We report data that support the distinction between childhood-onset and adolescent-onset type conduct problems. Natural histories are described from a representative birth cohort of 457 males studied longitudinally from age 3 to 18 years. Childhood- and adolescent-onset cases differed on temperament as early as age 3 years, but almost half of childhood-onset cases did not become seriously delinquent. Type comparisons were consistent with our contention that males whose antisocial behavior follows a life-course-persistent path differ from males who follow an adolescence-limited path. As adolescents, the two types differed on convictions for violent crime, personality profiles, school leaving, and bonds to family. These differences can be attributed to developmental history because the two groups were well matched on measures of antisocial conduct at age 18 years: parent-reports, self-reports, and adjudication records. By age 18 years, many conduct-problem boys had encountered factors that could ensnare them in an antisocial future: substance dependence, unsafe sex, dangerous driving habits, delinquent friends, delinquent perceptions, and unemployment. Implications for theory, research design, prevention, and therapeutic treatment of conduct problems are highlighted.