This book provides a comprehensive, organized, and concise overview of xenobiotic metabolic enzymes and their health implications. The subjects addressed are broad in scope with an emphasis on recent advances in research on biochemical and biomedical aspects of these enzymes. The xenobiotics discussed include not just drugs, but also food, smoke, and other environmental chemicals. The subjects covered in this work include: metabolic enzymes and their catalyzed reactions, reactive intermediates generated from metabolic activation, oxidative stress mediated by electrophilic reactive intermediates, bioactivation - mediated cellular and functional damages, activation of Nrf2 - ARE pathway, genetic variations affecting metabolic enzyme expression, enzyme polymorphisms affecting xenobiotic - mediated toxicity, induction of metabolic enzymes for health benefits, and a diversity of metabolic enzyme modulators.