The wasting away of tissue from disuse, denervation, or lack of nutrients, the body’s way of dismantling what it no longer needs or cannot maintain. Muscle atrophies within days of immobilization, the gut lining thins during prolonged fasting or intravenous feeding, and in starvation the body consumes its own muscle protein to keep vital organs running. The process is reversible when the cause is removed: refeeding rebuilds tissue, exercise restores muscle, and the gut lining regenerates remarkably fast. In nutrition, atrophy is the visible endpoint of deficiency, from the shrunken muscles of protein-energy malnutrition to the thinned skin of chronic illness. Its study teaches a fundamental principle: tissues are maintained dynamically, and continuous nourishment and use are the price of keeping them.