Aging is the sum of the deleterious effects of time upon the cellular function, microanatomy and physiology of each body system. These biological aging changes manifest in progressive deteriorations in physical condition, organ function, mental function, and immune response, but not necessarily correlating with the patient’s actual chronological age. How old an organism is in actual time is referred to as “chronological” aging and should be distinguished from “biological” aging," the relative functional age of each of an individual’s diverse organ systems. Because there is little correlation between biological and chronological age, each animal must still be evaluated as an individual.