Drug metabolites and their disposition in vivo are well recognized by scientists, clinicians, and regulatory agencies to be important when evaluating a new drug entity. In the past several decades, increased attention has been placed on drug metabolism for several reasons. Firstly, the number of drugs with active metabolites, by design (i.e., prodrugs) or by chance, has increased (1–3). This is exemplified by the transition from terfenadine to its active metabolite fexofenadine (3) and interest in the contributions of morphine-6-glucuronide (M6G) toward the analgesic activity of the age-old drug, morphine, and potential development of this active metabolite (4). In addition, with the advent of methods to establish the metabolic genotype and characterize the phenotype of individual patients (5,6) and the identification of specific isoforms of enzymes of metabolism, there is an increased appreciation of how elimination of a drug by metabolism can influence drug bioavailability and clearance, and ultimately affect its efficacy and toxicity. These rapidly evolving methods can be translated to permit cost-effective individual optimization of drug therapy on the basis of a subject’s metabolic capability (5,6), just as renal creatinine clearance has been used for years to assess renal function and permits individualized dose adjustment for drugs cleared by the kidney (7). Finally, the well accepted, though still poorly understood role of bioactivation in the potential toxicity of drugs and other xenobiotics (8,9) requires that metabolites continue to be evaluated and scrutinized for possible contributions to adverse effects observed in vivo. Though the importance of drug metabolism is seldom questioned, the interpretation and use of pharmacokinetic data on the disposition of metabolites is not well understood or fully implemented by some investigators. The objective of this chapter is to provide a basis for the interpretation and use of metabolite pharmacokinetic data from preclinical and clinical investigations.