Auxins, a group of plant signalling compounds, ensure proper growth and development of the plant in relation to both external and internal stimuli. Within a plant, auxin is distributed asymmetrically, thus creating local auxin maxima and minima. Such asymmetric auxin distribution underlies many developmental and stress adaptation processes and facilitates their spatial and temporal coordination. The appropriate local auxin level is achieved by regulation of biosynthesis, metabolism and through active, cell-to-cell transport relying on auxin carriers that are embedded in the plasma membrane. Owing to the physical–chemical properties of the auxin molecule, the asymmetric (polar) localisation of auxin efflux carriers, analogous in neighbouring cells, determines the direction of intercellular auxin flow (polar auxin transport). With respect to the important function of polar auxin transport for plant development, the polar auxin transport machinery is subject to a tight control at multiple regulatory levels.