Auxin acts as a general co-ordinator of plant growth and development, transferring information over both long and short ranges. Auxin famously appears to be extraordinarily multi-functional, with different cells responding very differently to changes in auxin levels. There has been considerable progress over recent years in understanding how this complexity is encoded in the cellular auxin response machinery. Of central importance is an elegantly short but versatile signaling pathway through which auxin triggers changes in gene expression. However, it is increasingly clear that this pathway is not sufficient to explain all auxin responses and other auxin signaling systems are emerging.