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Excretion is the biological process by which metabolic waste products and other non-useful materials are eliminated from an organism. These waste products are often toxic or can accumulate to harmful levels if not removed. Excretion is essential for maintaining homeostasis, the stable internal environment of the body.

The primary organs of excretion in humans are the kidneys, which filter waste products from the blood and produce urine. Urine is composed of water, urea (a byproduct of protein metabolism), salts, and other dissolved substances that are then eliminated from the body through the urinary tract. Other excretory organs include the skin, which eliminates sweat containing salts and urea, and the lungs, which expel carbon dioxide, a waste product of cellular respiration.

In plants, excretion is less pronounced, as they often reabsorb or utilize waste products. However, some plants excrete excess salts through specialized glands or by shedding leaves. Unicellular organisms typically excrete waste directly into their surroundings through their cell membranes. The mechanisms and specific waste products involved in excretion vary widely across different species and biological systems.