Over the past 20 years, numerous examples of a concerted action between drug metabolising enzymes and transporters in the detoxification of drugs have been identified. Several mechanisms can be used to explain this interplay. First, uptake and efflux transporters can modulate the exposure of a drug to metabolising enzymes within the cell. There can also be a substantial overlap in substrate specificity between transporters and enzymes. Moreover, specific regional expression of efflux transporters and enzymes, as observed in the gut, results in an increased potential for drugs to interact with the metabolising enzymes, thereby decreasing their systemic exposure. Finally, transporters can regulate the exposure of xenosensors to various inducers, impacting on the expression of both enzymes and transporters. The present chapter aims to highlight the impact of the enzyme–transporter interplay on the absorption, distribution and disposition of both new and existing drugs. The various approaches published to date, in order to understand and predict the impact of this interplay on pharmacokinetics, are described and their limitations discussed. Finally, challenges in predicting the risk of drug–drug interactions for compounds with a complex disposition and the limitations of the methods currently available will also be reviewed.