The study provides mixed support for Havighurst's thesis that successful achievement of developmental tasks leads to happiness and success with later developmental tasks. Although developmental progression in adolescence was positively related to happiness, as Havighurst predicted, the attainment of tasks was linked to a time-related happiness over the short term or at least within the particular developmental stage successfully negotiated. Gender was also a factor, in that the link between developmental status and later positive self-esteem was much stronger for males than for females. The results clearly show an association between developmental status and happiness within specific time frames; however, positive self-esteem at earlier developmental stages was the most enduring factor from adolescence into young adulthood, and it was more predictive of long-term inner satisfaction (happiness) than the achievement of specified developmental tasks. One of the difficulties of testing Havighurst's theory was the difference between his listing of developmental tasks for various age-related periods and the actual occurrence of these tasks for the current sample. For the current sample, the developmental tasks of later adolescence listed by Havighurst occurred much earlier; whereas, the developmental tasks of young adulthood, such as marriage and child-rearing, occurred later than indicated by Havighurst. This suggests a revision of the measurement of age-related developmental achievements in interaction with self-esteem and symptoms of maladjustment. The sample size was 125 for the 4 adolescent waves of measurement of self-esteem and symptoms of maladjustment (ages 14-17); and the sample size was 102 for the 2 waves of measurement of the same variables in young adulthood (ages 21-23). 4 tables, 1 figure, and 55 references