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 progresses through distinct stages as people age, such as early adulthood, middle adulthood, and late adulthood. 3. Life events theorists like Neugarten and Baltes et al. suggested that major life occurrences like marriage, death of a spouse, or historical events can influence development. 4. Transitions theorists like Bridges and Sugarman framed development as a series of transitions between stages, each involving endings, neutral periods, and new beginnings.