Plant development is characterised by a remarkable adaptability to different environmental conditions. Plants have the ability for extensive post-embryonic development involving the activity of stable stem cell populations (meristems), de novo organ formation and changes in growth direction. This provides exceptional flexibility in terms of growth and survival. Many of these developmental events are characterized by an asymmetric distribution (gradient) of the plant signalling molecule auxin. Auxin is unique among plant signalling molecules, because it is transported in a polar fashion through plant tissues. This transport is driven by the action of influx and efflux carrier proteins represented by the AUX1/LAX and PIN protein families, respectively. The subcellular localisation of PIN efflux carriers determines the direction of auxin flow, a mechanism, which can also define asymmetric auxin distribution within tissues. The auxin transport system thus provides positional and directional information for many aspects of adaptive plant development. It also represents a unique model system to study the functional link between basic cellular processes, such as vesicle trafficking or cell polarity establishment, and their developmental outcome at the level of the multicellular plant organism.