Polar auxin transport is essential for normal plant growth and development. Auxin moves between plant cells through a combination of membrane diffusion and carrier-mediated transport. Recent studies have identified several classes of membrane proteins involved in auxin transport, and have started to uncover a system that regulates auxin flux through plant tissues via the subcellular asymmetric locali- zation of these proteins. Localization of auxin transporters is controlled by vesicle-trafficking system, by interactions with protein kinases and phosphatases and by the profile of phospholipids and sterols in cell membrane.