The connective tissue matrix, within and around which the parenchymal cells carry on their daily activities, undergoes distinct changes with aging. Particularly noteworthy is the relative increase of connective tissue in various organs. The parenchymal cells react to the aging process by gradually decreasing their number in the various organs, a process leading to a decrease of total reserve functional capacity within the organism. Genetic mechanisms appear to establish the maximal attainable life span. Environmental factors determine how closely the organism approaches this maximum.