The authors describe the influence of childhood and adolescent physical activity on adult physical activity attitudes and behaviors. They conducted one-on-one, semistructured, indepth interviews with 16 men and 15 women from a longitudinal growth and development study and a follow-up investigation 25 years later. They used thematic analyses to analyze the data. Three themes emerged from the men's interviews: significant others, size and maturation, and physical ability, with clear distinctions among the active, average, and inactive men. The themes transitions, body image concerns, and significant others influenced the active, average, and inactive women. Specific relationships, circumstances, and attitudes formed in childhood and adolescence influenced adult physical activity predilection and behaviors.