The plant hormone and signaling molecule auxin is a key player during pattern formation, organogenesis, and various physiological processes. Recent discoveries in auxin biology point toward an auxin pathway with a higher complexity than previously anticipated. This prompted us to review this constantly growing field and to put these novel and exciting findings into a broader developmental and evolutionary context.Auxin signaling can be divided broadly into three layers that contribute to its complexity: the spatio-temporal pattern of its biosynthesis, its directional transport, and cell- or tissue-specific responses. In Arabidopsis thaliana, auxin is synthesized via several pathways in embryos, leaves, and roots (reviewed in Woodward and Bartel, 2005). The active transport of auxin, leading to the establishment of auxin maxima and gradients, ismediated by specific proteins: PIN-FORMED (PIN) efflux carriers, AUXIN1/LIKE AUX1 (AUX1/LAX) influx carriers, and MULTIDRUG RESISTANCE/P-GLYCOPROTEIN transporters (reviewed in Vieten et al., 2007). Upon reaching specific threshold concentrations, auxin induces specific responses that trigger diverse developmental and physiological effects (reviewed in Teale et al., 2006).The abundant interlinkages between these layers and thevariety of protein complexes involved in auxin-related processes contribute to the overall complexity of the auxin pathway. In addition, there are numerous inputs, intrinsic and environmental cues, that feed into this pathway, and this is quite frequently mediated by other hormonal pathways. Although the auxin pathway is being unraveled bit by bit, we will illustrate that there is still a long way to go before it will be fully understood.