The book consists of 8 chapters; it first provides an introduction on biotransformation of xenobiotics, and then presents modern approaches and strategies for dealing with metabolite characterization, using tools such as LC-MS, H-D exchange, stable isotopes LC-MS-NMR, and radiolabeled compounds. Also, strategies for dealing with reactive intermediates in drug discovery and development are presented as well as case studies on improving the decision-making process in the structural modification of drug candidates. The last chapter discusses the regulatory perspectives of safety testing of drug metabolites and why, how, and when to test their safety.