The aging process is difficult to analyze because of the way that the body's organ systems work together. The breakdown of one structure will ultimately affect the function of others. The medical field of gerontology deals with examining the biological changes of aging, both passive and active, that occur at the molecular and cellular levels. This paper will seek to explore those changes, and the affect that they have on the process of aging. Aging as a passive process involves the breakdown of structures and the resulting slowing of functions. At the molecular level, passive aging is seen as the degeneration of the elastin and collagen proteins of connective tissues. These proteins are primarily responsible for the smoothness and firmness of young skin. Consequently, when these proteins breakdown, the skin will sag, and the muscle will lose its firmness. Another sign of passive aging is the breakdown of lipids at the biochemical level. As aging membranes leak during this lipid degeneration, a fatty, brown pigment known as lipofuscin accumulates. As this happens, the mitochondria, or the "powerhouse of the cell" begins to break down, thereby decreasing the amount of energy that is being supplied to the cell. This cellular degeneration may be set into action by highly reactive chemicals known as free radicals. These molecules have an unpaired electron in the outermost valence shell. This causes the molecule to grab electrons from other molecules, setting into motion a chain reaction that destabilizes them, and causes death of the cell. These free radicals are the normal by-product of metabolism, and can also form due to exposure to radiation or toxic chemicals. Enzymes that usually inactivate these free radicals diminish in later years, thus increasing their danger. The active process of aging involves new activities or the appearance of new substances. This process actually begins at birth, as certain cells begin to die. As the cells die, old structures are destroyed, and new ones will take their place. The number of neurons in the fetal brain, for example, is halved as only certain ones are spared from death. The appearance of lipofuscin may also be considered an active process of aging, although it results from the passive breakdown of lipids. Another example would be autoimmunity, in which the immune system turns against the body, attacking its own cells as though they were harmful invading organisms. While the physical changes that the body undergoes due to aging will vary from person to person, certain evidences can be expected to appear in specific phases of life. As mentioned before, brain cells will begin to die at birth. By the age of 10, the thymus has begun to shrink. Sexual maturity occurs in the teens, while the peak of male sexuality will be in the early twenties. Peak muscle strength will be noted around the age of 25, while peak hair thickness is in the thirties. Also in the thirties: the skin becomes less elastic, the heart muscle thickens, and there is a decline in hearing and height, and the peak of female sexuality occurs. In the forties, the back slumps; there is an increase in weight; further decrease in height; and thinning, graying hair. The fifties may bring about menopause, as well as a decrease in taste, weight, metabolism, and memory. There is also an increased risk of diabetes during these years. While aging is not something that most people look forward to, it is something that can de done with grace and style, and can be enjoyed. Living a healthy, well- balanced lifestyle in the younger years can pave the way for many years of happiness later in life. Time to travel if one wants, time with a spouse, time with grandchildren, a better understanding of oneself; these are all benefits that the aging process can afford. And those are things that can be looked forward to and enjoyed.