Cellular functions are dependent on the import and export of molecules. Although some low-molecular-mass compounds by themselves can diffuse through biological membranes, the vast majority of the molecular traffic is assisted by transport proteins. Passive diffusion through pore-forming proteins as well as active transport driven by chemical gradients or ATP is crucial to metabolism as well as the biotransformation of xenobiotics. Pharmacotherapeutic effects are critically reliant on the uptake and the disposition of drugs in target tissues, and resistance mechanisms may involve not only enzymatic biotransformation but also cellular export of the active compound.