This 20-year prospective longitudinal study focuses on the contribution of mastery motivation and self-regulation to adult outcomes for individuals with Down syndrome. In earlier phases of the research, 25 participants completed measures of cognitive ability, mastery motivation and self-regulation in childhood (4 to 6 years) and adolescence (11 to 15 years). In the adult phase reported here, self-determination and adaptive behavior were assessed in 21 of the original participants at age 23 to 26 years. Mastery motivation and self-regulation made unique contributions to adult outcomes, over and above the effects of cognitive ability. The findings provide powerful evidence about the important role of child and adolescent mastery motivation and self-regulation for the adult lives of individuals with Down syndrome.