Drug transporters play an important role in the absorption, distribution, excretion and toxicity of both endogenous and exogenous compounds. Transporters may act as physiological 'gatekeepers' in the regulation of the pharmacological and/or toxicological effects of drugs by limiting distribution to tissues responsible for their effect and/or toxicity. This review will first provide a brief outline of the characteristics of membrane bound drug transporter families and their respective roles in regulating drug pharmacokinetics. This background then provides the context for a discussion of the characterization of a drug candidate as a substrate, inhibitor and/or inducer of drug transporter(s), followed by an assessment of the in vitro and in vivo preclinical methods used in drug discovery and development for screening molecules to identify potential transporter interactions. Finally, specific examples of the translation of in vitro findings to the in vivo effects are discussed to link the current understanding of the impact of drug transporters to clinical pharmacology. Thus, the goal is to provide the drug discovery scientist with a cadre of concepts, strategies, and tools for ultimately making rational decisions in drug design and delivery resulting in the optimization of drug concentrations at the target of pharmacology.