Plants experience various biotic and abiotic stimuli including microorganisms, insects, light, temperature and other environmental stresses. Plants use a group of chemical messengers called hormones to respond to these various stimuli to have proper growth and development. Hormones, including auxin, promote, inhibit, or qualitatively modify plant growth and development. The characterization of auxin-related mutants in hormone response pathways provides not only a great opportunity to understand the action of this hormone in plant physiology and development but also helps to dissect the molecular genetics of auxin metabolism and signaling pathways, and the isolation of the corresponding genes. This paper summarizes the knowledge on the biosynthesis and metabolism of auxin and updates our understanding of the molecular mechanisms of auxin signaling based on-an analysis of recent progress.