Normal aging is an artificial concept which describes physiologic changes that occur with advancing age. Many researchers have tried to find out the cause of aging. Two kinds of theories have been postulated to explain why aging occurs: (1) Theories of aging based on intrinsic factors and (2) Theories of aging based on extrinsic factors. A sound knowledge of features of normal aging provides a baseline against which a thorough evaluation of elderly patients can be carried out. The features of normal aging, however, differ from one physiologic system to another; hence they should be discussed system wise. Accordingly, in this chapter Physiological response to various systems such as musculoskeletal system, cardiovascular system, pulmonary function, nervous system, special senses, vestibular system and other systems has been discussed briefly. However, aging is not that simple; it is rather far too complex and probably does not have a single cause. No matter how complex, physical therapist should use this existing knowledge of aging process to understand thoroughly how to help elderly achieve functional independence.