Auxin is a simple molecule but one with a complex and crucial influence on plant development. Accumulation and response to this important plant hormone underlies events as diverse as embryo patterning and growth responses to light and gravity. As such, research on auxin can be traced back to Darwin and has flourished into an immense body of work that has often had implications beyond plant biology. The latest instalment of the auxin story is no different:(1) the solution of the crystal structure of the auxin receptor TIR1 illustrates, in spectacular detail, precisely how auxin is perceived and provides an insight into the working of a new class of receptor, which seems likely to be the first example of a new paradigm in eukaryotic signal transduction. BioEssays 29:953–956, 2007. © 2007 Wiley Periodicals, Inc.