Despite advances in the knowledge of tooth morphogenesis and differentiation, little is relatively known about the aetiology and molecular mechanisms underlying supernumerary tooth formation. Mice, which are commonly used for studying tooth development, only exhibit one dentition. However, a number of mouse models are now starting to provide some insight into the mechanisms that control overall tooth number within the dentition. This review describes recent advances in our understanding of the molecular mechanisms of supernumerary tooth formation. Indeed, many of the molecular signaling pathways known to be involved in normal development of the tooth germ can also give rise to supernumerary teeth if inappropriately regulated. These include components of the Hedgehog, FGF, Wnt and BMP families, which may potentially play a role in human supernumerary tooth formation.