The study of biological aging has seen a spectacularly fast progress in the last decade. Besides a better understanding and comprehension of physiological aspects, an important advance has been the identification of at least a hundred different genes which control the process of aging. Their mechanism of action falls within the expectations from a handful of theories which attempt to provide a global explanation of the phenomenon of aging, including free radicals, cell senescence and loss of regenerative capacity through the activation of stem cells. In this review we will concentrate in these biological aspects, with a special emphasis on animal models used to study both the genetics and physiology of aging as well as experimental approaches to test the aforementioned theories. It should be emphasized that, while the emphasis is in purely biological aspects of the process, the fast pace of aging of the world's population, including Chile, needs a rapid advance also in our understanding o fits social and economic implications.