1) Adult development theorists propose that development occurs through age/stage theories, life events, or transitions between stages. 2) Age/stage theorists like Levinson and Erikson proposed that development occurs in distinct stages as people age, such as early adulthood, middle adulthood, and late adulthood. 3) Life events theorists like Neugarten and Baltes proposed that major life events like marriage, death of a spouse, or historical events like wars trigger development. 4) Transitions theorists like Bridges and Sugarman proposed that development occurs through transitions between stages, which involve an ending phase, neutral zone, and new beginning.