The characterization of a drug’s metabolites plays a crucial role throughout its development. In the early discovery phase, studies are designed to help the medicinal chemist quickly identify metabolic “hot spots” that might require chemical modification in order to improve pharmacokinetics. At this early stage, the emphasis is typically on analyses with fast turnaround speeds, due to the large number of compounds involved. As the development process moves forward and the number of drug candidates is reduced, the emphasis shifts from speed to completeness, especially when regulatory and toxicity issues are involved. By the time a drug candidate reaches the latter stages of development, a team of scientists might spend years exhaustively characterizing its metabolic products.