Over the past few decades, a tremendous amount of work has been done on the molecular characterization of transport proteins in animals and humans, leading to a better understanding of the physiological roles of a number of transport proteins. Furthermore, there is increasing preclinical and clinical evidence to support the importance of transport proteins in the pharmacokinetics and toxicokinetics of a wide variety of structurally diverse drugs. As a consequence, the degree of expression and functionality of transport proteins may directly affect the therapeutic effectiveness, safety and target specificity of drugs. Recently, there has also been increased awareness about potential drug-drug, drug-herb and drug-food interactions involving transporters. Traditionally, a change in metabolic clearance of a drug, particularly via cytochrome P450-mediated metabolism, has been considered the cause of many clinically important drug interactions. However, increasing evidence suggests that some drug interactions result from changes in the activity and/or expression of drug transporters. Accordingly, assessment of the clinical relevance of transporter-mediated drug interactions has become a regulatory issue during the drug approval process and also the evaluation of drug interaction potential has become an integral part of risk assessment during drug development processes. Therefore, this review will highlight the role of some selected drug transporters in drug interactions, as well as their clinical implication.