A remarkably diverse family of glutathione S-transferases (GSTs) has evolved in higher organisms. These intracellular enzymes catalyze the nucleophilic attack of glutathione on a variety of hydrophobic, electrophilic xenobiotics often resulting in conjugated or transformed metabolites that are less toxic and more easily excretable. Additionally, some GST isozymes may participate in the repair of oxidative damage to membrane lipids and DNA. Finally, GSTs are high capacity intracellular binding proteins which, independently of their enzymatic activities, may serve in the storage, transport, or sequestration of many hydrophobic compounds. These properties suggest that GSTs may function as important cellular defenses against the cytotoxic effects of carcinogenic and antineoplastic agents. Here we discuss recent evidence that bears upon this notion.