Ectodermal dysplasias are heterogeneous heritable conditions characterized by congenital defects of one or more ectodermal structures and their appendages. Of approximately 200 different ectodermal dysplasias, about 30 have been identified at molecular level with identification of the causative gene. Itin and Fistarol emphasized that rather commonly non-fully expressed phenotypes exist, which make a clinical diagnosis more difficult. Freire-Maia and Pinheiro used the clinical aspects for their classification and Priolo integrated molecular genetic and clinical aspects for her scheme. Those two more historical classification schemes have the difficulty that when applied strictly, several additional groups of disorders should be integrated within the term of ectodermal dysplasias, for example, keratodermas with skin or hair alterations or the ichthyoses with associated abnormalities. Such consequent classification would lead to an endless list of conditions and would be useless for practical work. Recent evidence implicates a genetic defect in different pathways orchestrating ectodermal organogenesis. Modern molecular genetics will increasingly elucidate the basic defects of the different syndromes and yield more insight into the regulatory mechanisms of morphogenesis. In this way a reclassification of ectodermal dysplasias will be possible according to the function of their involved mutated genes. I will focus on the fact that with molecular methods it is possible to diagnose oligosymptomatic forms of ectodermal dysplasia. This is much more common than earlier anticipated and with the classification of ectodermal dysplasia on the basis of molecular diagnosis a new avenue is opened for symptom complexes which were impossible to classify in former times. © 2009 Wiley-Liss, Inc.