Introduction: In all multicellular organisms extensive communication is required between cells and tissues in order to coordinate growth and development. Both plants and animals have signaling chemicals, traditionally termed hormones that mediate short- and long-distance communication. The first phytohormone to be discovered was auxin, a substance that has since been implicated in an increasingly wide varietyof developmental processes. It has emerged from decades of studies that a unique property of auxin - its directional movement between cells (polar auxin transport) - is a crucial process in auxin biology. In this report, we will first review the key events in the history of auxin research and then explore the current concepts and latest findings of the mechanisms of polar auxin transport and the resulting asymmetric auxin distribution.©2007 American Society of Plant Biologists.