In the early phase of preclinical drug development it is important to conduct in vitro/in vivo experiments on the metabolism of new chemical entities in order to predict bioavailability and to clarify metabolic profiles and pathways. This work is fundamental because drug metabolism influences drug efficiency and safety. The flavin-containing monooxygenases (FMOs) is one of the major classes of microsomal monooxygenases responsible for the metabolism of nucleophilic heteratom-containing drugs. This book provides a summary of recent scientific results regarding FMO and provides information about its biochemical, structural and catalytical properties; isoforms; substrate specificity; physiological significance and toxicological importance; relevance in drug metabolism/development; and regulation. The issues discussed in this book should be especially useful for students of the Faculty of Pharmacy and Medicine in order to gain a wider perspective of the field of drug metabolism and for professionals working in the field of Absorption, Distribution, Metabolism, Excretion, and Toxicity (ADMET) science.