Essential amino acids, including methionine (a sulfur-containing amino acid), must be supplied through the diet, as humans and animals, except for ruminant animals, are unable to synthesize them. Sulfur is present in body tissues as part of the amino acids: methionine, cysteine, and taurine. The relative reduction-oxidation state of the cell depends primarily on the precise balance between 86concentrations of reactive oxygen species (ROS) and the cysteine-dependent (thiol) antioxidant buffers, glutathione (GSH) and thioredoxin. These antioxidants have high affinity for ROS, thus protecting other intracellular molecules from oxidative damage. Dietary deficiency of sulfur is relatively uncommon, whereas its toxicity is increasingly recognized as a serious concern in our environment. Human activities, such as burning fossil fuels, refining petroleum, smelting sulfur compounds of metallic minerals into free metals, and other industrial processes, have a large impact on the atmospheric sulfur balance. The atmospheric sulfur is lost into water reservoirs, where some of the sulfur enters marine communities and soil as it moves through the food chain. The metabolic product, sulfur dioxide (SO2), is thus considered one of the major air pollutants worldwide. Sulfur toxicity is recognized to be a consequence of metabolic derangement of methionine. Its metabolic product, homocysteine, is thought to be an independent risk factor for arterial diseases.