In the previous chapter, we examined some of the cultural and contextual factors impacting development in late adulthood. This stage of life is typically characterized as starting in the mid 60s and continuing into the 80s, 90s, or until the end of the natural lifespan. In this chapter, we look at developmental theories, models, and research about older adults, and strive to understand how to apply these theories to work with adults in late adulthood. As it is important to always view developmental theories with a critical eye, we include both critiques of the theories presented, and recent and relevant research and writings about how these theories inform our understanding of older adults. Using the case of Rose, our fictional client introduced in the previous chapter, we will present psychosocial development theory (Erikson), human potential stages (Cohen), bioecological theory (Bronfenbrenner), and the ecological theory of aging (Lawton). We include additional cultural and contextual factors of aging impacting development, and conclude with thoughts from two experts from the field, Dr. William Barkley and Dr. Nina Nabors.