The book summarizes what is meant by theory, and why theory is so important to advancing aging-related research, policy, practice, and intervention, and can keep researchers and practitioners in gerontology abreast of the newest theories and models of aging. It addresses theories and concepts built on cumulative knowledge in four disciplinary areas, biology, psychology, social sciences, and policy and practice, as well as landmark advances in trans-disciplinary science. Since longevity is indirectly governed by the genome it is sexually determined, and because aging is a stochastic process, it is not. Chapters cover major paradigm shifts that have occurred in geropsychology, theories in the sociology of aging, evolutionary theories pertaining to human diseases, theories of stem cell aging, evidence that loss of proteostasis is a central driver of aging and age-related diseases, theories of emotional well-being and aging, theories of social support in health and aging, and other theories such as environmental gerontological theories and biodemographic theories. Many chapters also address connections between theories and policy or practice. The book also contains a new section, "Standing on the Shoulders of Giants", which includes personal essays by senior gerontologists who share their perspectives on the history of ideas in their fields, and on their experiences with the process and prospects of developing good theory.