The homeostasis of endogenous organic cations, such as choline and N P-methylnicotinamide, monoamine neurotransmitters, cationic drugs, such as cimetidine, morphine, quinine and amantadine, and of cationic xenobiotics is controlled by reabsorption and excretion in the small intestine, by metabolic conversion and excretion in the liver, and by reabsorption and excretion in the kidney [1–6]. In the kidney organic cations may be ultrafiltrated in the glomeruli and reabsorbed or secreted in renal tubules. Hydrophilic cations are readily ultrafiltrated, however, more hydrophobic cations are bound to plasma membrane proteins that may not permeate the filtration barrier. Secretion and reabsorption of organic cations have been described in renal proximal tubules but may also occur in distal tubules or collecting ducts [7–11]. These processes have been functionally characterized by measurements using intact kidneys or tissue slices [10, 12–14], by microperfusion experiments using proximal tubules [7, 9, 15], by uptake measurements using isolated tubules or cells [8, 11, 16, 17], and by uptake measurements using membrane vesicles from proximal tubules [18–28]. In luminal and basolateral membranes of proximal tubules different transport processes have been demonstrated, although all the involved transport systems may not have been identified by these measurements because a variety of different polyspecific organic cation transporters with overlapping substrate specificity are expressed [5]. After the cloning of the first organic cation transporter rOCT1 from a rat kidney library [29], an increasing number of homologous cation transporters have been identified and it has been shown that the renal anion transporters also belong to this new protein family. The functional characterization of the expressed transporters and their immunohistochemical localization in the kidney has begun [30–35]. This review describes the molecular structure of renal organic cation transporters and homologous gene products. The transport properties and renal localization of the cloned cation transporters are summarized and their presumed role in renal cation handling of endogenous and exogenous cations is discussed.