Polar transport of auxin is essential for normal plant growth and development. On a cellular level, directional auxin transport is primarily controlled by an efflux carrier complex that is characterized by the PIN-FORMED (PIN) family of proteins. Detailed developmental studies of PIN distribution and subcellular localization have been combined with the analysis of changes in localized auxin levels to map PIN-mediated auxin movement throughout Arabidopsis tissues. Plant orthologs of mammalian multidrug-resistance/P-glycoproteins (MDR/PGPs) also function in auxin efflux. MDR/PGPs appear to stabilize efflux complexes on the plasma membrane and to function as ATP-dependent auxin transporters, with the specificity and directionality of transport being provided by interacting PIN proteins.