In the coming years, clinical psychologists will provide services to increasing numbers of older adults, owing in part to the greater percentage of older adults in the United States. The Pikes Peak Model of Training in Professional Geropsychology includes General Knowledge about adult development and aging as a foundation for working with older adults. The current article summarizes demographics of the aging population, diversity in the aging process, research methods, and current theories of aging. Age-related changes in physical health, social development, personality, and emotional development are also reviewed. Knowledge about adult development and aging will facilitate case conceptualization and treatment planning, as well as contextualizing older adults’ presenting problems within broader changes that take place with aging.