Transporters are membrane-spanning proteins that facilitate the movement of lipids, sterols, ions, small molecules, drugs, amino acids, sugars and large polypeptides across biological membranes. Transport can be passive or active. Passive transport follows a concentration gradient so requires no energy input and is further divided into two types - diffusion, which is spontaneous movement across the membrane and facilitated transport. In contrast active transport requires energy to transport substances into a cell.

Membrane transporters can be also divided into three main classes; ABC transporters, P-type ATPases and the solute carrier family (SLC). ABC transporters are primary active transporters, which transport a wide range of substrates mainly to the outside of a cell membrane or organelle. The solute carrier family includes the biogenic amine transporters (NET, DAT and SERT) and the Na+/H+ exchanger.

Inhibitors of transporters have proved useful in the treatment of a variety of disorders, including depression (SERT), epilepsy (GABA transporter) and Parkinson's disease (DAT).