Aging inspired a large number of theories trying to rationalize the aging process common to all living beings. In this publication the most important environmental and intrinsic mechanisms involved in the aging process and in its pathological consequences are reviewed. Furthermore theoretical and experimental evidence of the most important theoretical elements based on Darwinian evolution, cellular aging, role of cell membranes, free radicals and oxidative processes, receptor-mediated reactions, the extracellular matrix and immune functions as well as the most important environmental and intrinsic mechanisms involved in the aging process and in its pathological consequences are discussed. These presentations of theories and related experimental facts give a global overview of up to date concepts of the biology of the aging process and are of essential reading not only for specialists in this field but also for practitioners of scientific, medical, social and experimental sciences.