Renal organic anion secretion has been implicated in numerous clinically significant drug interactions and adverse reactions, indicating the importance of a detailed understanding of this pathway for the development of optimum therapeutics. With the cloning of multiple genes encoding organic anion transporters (OATs), the study of organic anion secretion has entered the molecular age. In this review, we focus on various aspects of the molecular biology and pharmacology of the OATs, including discussion of their structural biology, genomic organization in pairs, developmental regulation, toxicology, and pharmacogenetics. We propose functional, pathophysiological, and evolutionary hypotheses to help explain recent experimental and genomic data.