Tooth agenesis denotes a condition where teeth are missing due to a developmental failure. According to severity, this malformation can be subdivided into hypodontia, oligodontia and anodontia. With a frequency of 20–30%, tooth agenesis is the most prevalent dental dysplasia, but not all teeth are equally affected. Familial occurrence and concordance in twins indicate potential genetic causes. Thus far, mutations could be identified for the genes MSX1, PAX9 and AXIN2. All result in oligodontia. It is assumed that the defects cause haploinsufficiency, reducing the functional gene products below a critical level, which is required for normal odontogenesis. Both hypodontia and oligodontia can occur as an isolated dental malformation or in combination with defects in other organs, for example the skin and its appendages. Such syndromes can result from genetic defects affecting MSX1 and the EDA signalling pathway. In the case of isolated hypodontia, associations have been found with sequence variants of several genes.