Vitamin C is an essential dietary nutrient required as a co-factor for many enzymes, and humans are among the few animals that lack the ability to synthesize the compound from glucose. The reduced form of the vitamin, ascorbic acid, is an especially effective antioxidant owing to its high electron-donating power and ready conversion back to the active reduced form. Concentrations of the vitamin in body tissues and fluids are regulated through interactions of intestinal absorption, cellular transport, and excretion. The amount of vitamin C needed to prevent scurvy is very small and easily obtained in nearly all Western diets. There is great interest in the clinical roles of vitamin C because of evidence that oxidative damage is a root cause of, or at least associated with, many diseases. Population studies show that individuals with high intakes of vitamin C have lower risk of a number of chronic diseases, including heart disease, cancer, eye diseases, and neurodegenerative conditions. However, these results may simply reflect a more healthful diet or lifestyle for individuals with a high vitamin C intake. At present, data from controlled clinical trials have not established that higher intakes of vitamin C alone will help prevent chronic degenerative diseases. However, the evidence that ascorbic acid acts as an important antioxidant in many body tissues is convincing. The new higher Recommended Dietary Allowance (RDA) for vitamin C of 75 mg for women and 90 mg for men is, for the first time, based on the vitamin's role as an antioxidant as well as protection from deficiency. In healthy people, amounts greater than the RDA do not appear to be helpful. Vitamin C nutriture may be more important for people with certain diseases or conditions. High intakes of the vitamin are generally well tolerated; a Tolerable Upper Level was recently set at 2 g based on gastrointestinal upset that sometimes accompanies excessive intakes.