Drug interactions in oncology are of particular importance owing to the narrow therapeutic index and the inherent toxicity of anticancer agents. Interactions with other medications can cause small changes in the pharmacokinetics or pharmacodynamics of a chemotherapy agent that could significantly alter its efficacy or toxicity. Improvements in in vitro methods and early clinical testing have made the prediction of potentially clinically significant drug interactions possible. We outline the types of drug interaction that occur in oncology, the mechanisms that underlie these interactions and describe select examples.