Most modern theories of aging have centered around the notion that age-related deterioration is primarily due to structural and functional modifications of cellular constituents. Among them are three currently popular hypotheses--the Free Radical, Glycation, and Maillard Theories of Aging. The first proposes that age-related effects are due to free radical reactions that damage cellular constituents, while the latter propose damage induced by nonenzymatic glycation and other Maillard reactions and the consequent modification of macromolecules as the primary cause of aging. Although these hypotheses were formulated independently, recent studies suggest that free radicals, glycation, and Maillard reactions may in fact represent partially interactive elements of a single, more complex biochemical pathway. We therefore propose the Free Radical-Glycation/Maillard Reaction Theory of Aging: that age-related deterioration is produced by the sum of the damage induced by free radicals, by glycation, by Maillard reactions, and by their interactions.