Plant growth and development is regulated by a structurally unrelated collection of small molecules called plant hormones. During the last 15 years the number of known plant hormones has grown from five to at least ten. Furthermore, many of the proteins involved in plant hormone signalling pathways have been identified, including receptors for many of the major hormones. Strikingly, the ubiquitin–proteasome pathway plays a central part in most hormone-signalling pathways. In addition, recent studies confirm that hormone signalling is integrated at several levels during plant growth and development.