This article presents findings about continuities in personality development that have been uncovered in the Dunedin study, an investigation of a cohort of children studied from age 3 to 21. At age 3, children were classified into temperament groups on the basis of observations of their behavior. In young adulthood, data were collected from study members themselves, from people who knew them well, and from official records. Undercontrolled 3-year-olds grew up to be impulsive, unreliable, and antisocial, and had more conflict with members of their social networks and in their work. Inhibited 3-year-olds were more likely to be unassertive and depressed and had fewer sources of social support. Early appearing temperamental differences have a pervasive influence on life-course development and offer clues about personality structure, interpersonal relations, psychopathology, and crime in adulthood.