This chapter presents an overview of intrapsychic theories, cognitive theories, behavioral and environmental theories, biological theories, and integrative theories. Past ideas about the nature of adolescent development serve as foundations for current adolescent developmental theories. In many ways, the adolescent years are the culmination of childhood; hence, in order to truly understand adolescence a review of what happens in the years leading up to adolescence can help clarify the nature of adolescents. Although the early biological process of puberty begins to develop several years before adolescence, in Freud’s theory puberty and adolescence are considered roughly equivalent. Adolescents experience a reawakening of and an obsession with sexuality. Studies indicate that occurrences of eating disorders, obsessive-compulsive patterns, and self-reports of same-sex attraction surface during the adolescent years as a result of the reawakening of the underlying subconscious conflicts.