Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply to the target organ, or disease intrinsic to the tissue itself. Hormonal and nerve inputs that maintain an organ or body part are referred to as trophic.

Atrophy is a general physiological process of reabsorption and breakdown of tissues, involving apoptosis on a cellular level. It can be part of normal body development and homeostatic processes, or as a result of disease. Atrophy resulting from disease of the tissue itself, or loss of trophic support due to other disease is termed pathological atrophy.

Examples of atrophy as part of normal development include shrinkage and involution of the thymus in early childhood and the tonsils in adolescence.

Atrophy of the breasts can occur with prolonged estrogen reduction, as with anorexia nervosa or menopause. Atrophy of the testes occurs with prolonged use of enough exogenous sex steroid (either androgen or estrogen) to reduce gonadotropin secretion. The adrenal glands atrophy during prolonged use of exogenous glucocorticoids like prednisone.

Disuse atrophy of muscles, with loss of mass and strength, can occur after prolonged immobility, such as extended bedrest. This type of atrophy can usually be reversed with exercise. Astronauts must exercise regularly to prevent atrophy of their limb muscles while they are in zero gravity.

Pathologic atrophy of muscles can occur due to diseases of the motor nerves, or due to diseases of the muscle tissue itself. Examples of atrophying nerve diseases include poliomyelitis, amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease), and Guillain-Barre syndrome. Examples of atrophying muscle diseases include muscular dystrophy, myotonia congenita , and myotonic dystrophy.