The plant hormone auxin is frequently observed to be asymmetrically distributed across adjacent cells during crucial stages of growth and development. These auxin gradients depend on polar transport and regulate a wide variety of processes, including embryogenesis, organogenesis, vascular tissue differentiation, root meristem maintenance and tropic growth. Auxin can mediate such a perplexing array of developmental processes by acting as a general trigger for the change in developmental program in cells where it accumulates and by providing vectorial information to the tissues by its polar intercellular flow. In recent years, a wealth of molecular data on the mechanism of auxin transport and its regulation has been generated, providing significant insights into the action of this versatile coordinative signal.