Supernumerary teeth are a common clinical and radiographic finding and may produce occlusal and dental problems. The aetiological basis of extra teeth is poorly understood in human populations; however, the mouse provides a useful model system to investigate the complex genetics of tooth development. This article describes recent advances in our understanding of the genetic basis of supernumerary teeth. We have reviewed biological evidence that provides insight into why supernumerary tooth formation may occur. Indeed, many of the molecular signalling pathways known to be involved in normal development of the tooth germ can also give rise to additional teeth if inappropriately regulated. These include components of the Hedgehog, FGF, Wnt, TNF and BMP families, which provide a useful resource of candidate genes that may potentially play a role in human supernumerary tooth formation.