Mutations in members of the ectodysplasin (TNF-related) signalling pathway, EDA, EDAR, and EDARADD in mice and humans produce an ectodermal dysplasia phenotype that includes missing teeth and smaller teeth with reduced cusps. Using the keratin 14 promoter to target expression of an activated form of Edar in transgenic mice, we show that expression of this transgene is able to rescue the tooth phenotype in Tabby (Eda) and Sleek (Edar) mutant mice. High levels of expression of the transgene in wild-type mice result in molar teeth with extra cusps, and in some cases supernumerary teeth, the opposite of the mutant phenotype. The level of activation of Edar thus determines cusp number and tooth number during tooth development.