Technical Abstract: This chapter for the Enclyclopedia of Human Nutrition (3rd edition) summarizes the structure, chemical and physiological mechanisms, dietary sources, and metabolism of carotenoids. Carotenoids are a family of phytonutrients which have antioxidant properties under most physiological conditions. Numerous studies indicate that carotenoids and their metabolites play a role in combating oxidative damage and other degradative reactions that are harmful to human health. Most of these functions seem to be related to their antioxidant nature and ability to dissipate energy from light and free radical-generating reactions. Some carotenoids are also important sources of vitamin A, a nutrient essential for normal growth, development, and eyesight. Good dietary sources are orange, red, yellow, and dark green fruits and vegetables.