Human lung is a major target organ for all inhaled drugs, environmental toxicants and carcinogens. Recent hypotheses suggesting a role for environmental toxicants in the pathogenesis of lung diseases, such as lung cancer and chronic obstructive pulmonary disease have stimulated interest in research on the xenobiotic metabolizing capability of the lung. Many of the compounds associated with these diseases require enzymatic activation to exert their deleterious effects on pulmonary cells. Interindividual differences in in situ activation and inactivation of xenobiotics may contribute to the risk of developing of lung diseases associated with these compounds. The major xenobiotic metabolizing enzymes, including both phase I and phase II enzymes, have been detected in animal and human lung tissues. Although the lung cytochrome P450 (CYP) and other xenobiotic metabolizing enzymes share many common features with those present in other tissues such as liver, kidney and gut, there are some distinctive differences. It is evident from the studies carried out to date CYP1A1, 1B1, 2A13, 2F1, 2S1 and 4B1 are preferentially expressed in the lung together with CYP2E1 and 3A5. This review provides a detailed picture of major xenobiotic-metabolizing phase I (CYPs, epoxide hydrolases, flavin monooxygenases, etc.) and phase II enzymes (conjugation enzymes, including several transferases) expressed in human lung. The roles of individual metabolizing enzymes and their genetic polymorphisms are also discussed.