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.