This book summarizes the recent advances for the understanding of circadian clock system in the regulation of drug metabolism and pharmacokinetics. Basic knowledge in the field of circadian clock and pharmacokinetics are systemically introduced to make it easier for readers to understand the entire book’s contents. The rhythmic expression of DMEs (drug-metabolizing enzymes) and transporters are summarized, and the underlying mechanisms thereof (i.e., regulation by circadian oscillators) are discussed. Typically, evidence for the DME- and transporter-mediated chronopharmacokinetics, chronotoxicity and chronoefficacy are highlighted in this book.