This paper explores the nature of the aging process by providing an overview of the available evidence relating to the body systems that are most critical to biological function. Each system is treated separately to more clearly describe various aspects of the aging process and then integrated in a discussion of the theories of biological aging. Concepts are presented so they may be understood without a thorough knowledge of biological and chemical terminology. The issue of lifestyle intervention in the aging process is addressed and an examination is made of those components that have been shown to affect physiological aging. (Author/JD)