The tripeptide glutathione (GSH) has important antioxidant properties, scavenges free radicals, and serves as a cofactor for glutathione S-transferase conjugation of many xenobiotics. GSH is synthesized in two steps. The first and, often, rate-limiting step is the formation of γ-glutamylcysteine, which is catalyzed by the inducible heterodimeric enzyme glutamate cysteine ligase (GCL). The two subunits of GCL are the catalytic subunit (GCLC) and the modifier subunit (GCLM). In this unit, the generation and basic characterization methodologies of transgenic mouse models that have been developed to (1) conditionally over express both GCL subunits; (2) lack GCLM (Gclm null); and (3) create a hybrid between Gclm conditional over-expressing mice on a Gclm null genetic background are discussed. These models can be used to explore the fundamental role of GCLC and GCLM in GSH synthesis, as well as the toxicological role of GSH and its synthesis in xenobiotic metabolism and response to oxidative stress.