The ATP-binding cassette (ABC) protein superfamily is one of the largest known, with over 120 members in both Arabidopsis thaliana and rice (Oryza sativa). Most, but not all, ABC proteins are modularly organized membrane proteins ("ABC transporters") that mediate MgATP-energized transmembrane transport and/or regulate other transporters. The range of processes in which members of the various subclasses of plant ABC transporters have been implicated encompasses polar auxin transport, lipid catabolism, xenobiotic detoxification, disease resistance, and stomatal function. Although it is often possible to predict the likely function of a plant ABC transporter on the basis of its subfamily membership, there are many whose capabilities deviate from what would be predicted from the properties of even their most sequence-related counterparts. When taking account of this and the disparate processes in which the few that have been characterized participate, it is likely that elucidation of the mechanistic basis of any given plant process will necessitate consideration of at least one ABC transporter.