During the past decades, pharmacokinetics has been defined as the study of drug absorption, distribution, metabolism, and excretion (ADME), when the drug is introduced into a biological system, such as the human body. Pharmacokinetics is now challenged by the growing importance of transporters, a relatively new and potentially major factor in drug ADME. The recent intrusion of drug transporters means that there is no single mechanism by which drugs permeate through membranes. The presence of transporters in membranes modulates the traditional theory of 'diffusional pharmacokinetics' towards 'vectorial pharmacokinetics' in which ADME processes are governed more deterministically. Drug transporters are also clinically important. They can modulate the pharmacological activity of drugs by affecting their intracellular concentrations and causing toxicity in specific organs due to intracellular drug accumulation. Finally, they are key players in drug-drug interactions, where they are as important as the drug metabolizing enzymes.