Adult development theories can be categorized as age/stage theories, life events theories, or transitions theories. Age/stage theorists like Levinson and Erikson proposed that development occurs in distinct stages as people age. Life events theorists like Neugarten and Baltes et al. argued that development coincides with major life occurrences. Transitions theorists such as Bridges and Sugarman viewed development as a series of transitions between stages, each involving endings, neutral periods, and new beginnings.