It is now becoming increasingly clear that metabolism alone cannot adequately account for the observed intersubject variability in drug disposition or response. Carrier-mediated processes, or transporters, appear to play an important role in drug absorption, distribution and excretion. Thus, for many drugs, transport and metabolism must be considered together to better predict drug disposition in vivo. Accordingly, this review will outline relevant background information regarding drug transporters. In addition, in vitro drug transporter expression methodologies of potential relevance to the drug screening process will be covered in greater detail.