In an exhaustive treatise Walford has proposed an immunologic theory of aging which he claims provides a link between etiologic theories and actual aging in higher animals. We have re-examined the relationships between various aspects of the aging process and the functional state of the immune system and have classified aging into primary and secondary events. Primary aging would be that period of chronological aging during which there is a genetically programmed decline in functional effectiveness which is either associated with or controlled by the immune system. Either the thymus acts as a biological clock which is genetically programmed to operate at a rate consistent with the optimal life time of the species adhering to the Hayflick limit or the T cell maturation process decays subsequent to functional exhaustion of cells, most likely, but not necessarily residing in the thymus, which elaborate factors necessary for T cell maturation and function. All other concomitants of aging are classified as secondary aging events. These events would encompass pathology and disease processes commonly associated with aging but which have not been shown to have a direct causal relationship.