The topic of the inhibition of the enzymes of drug metabolism is of great interest to enzymologists, chemists, pharmacologists, and clinicians. Two major practical applications of this knowledge of inhibition are important in the pharmaceutical industry. One is drug-drug interactions, i.e., one drug may inhibit the biotransformation of another when two are taken concurrently (Correia and Hollenberg 2015, 224Guengerich 1997). Such interactions can be fatal, and the possibilities are scrutinized by regulatory agencies. The other major interest in enzyme inhibition is based on the selection of enzymes as targets for drug action (Yu et al. 2014). For instance, monoamine oxidase and some of the cytochrome P450 (P450) enzymes are targets because the products of their normal reactions can be deleterious under certain conditions (Guengerich 2017). However, the focus of this chapter will be on inhibition of drug metabolism as opposed to drug targets.