The ectodermal dysplasias (EDs) are a large and complex group of disorders. In various combinations, they all share anomalies in hair, teeth, nails, and sweat gland function. The anomalies affecting the epidermis and epidermal appendages are extremely variable. Many are associated with malformations in other organs and systems. Clinical overlap is present among EDs. Few causative genes have been identified, to date. Most of the EDs present multisystem involvement with abnormal development of structures also derived from mesoderm. In the last few years, it has become evident that gene expression in the EDs is not limited to the ectoderm and that there is a concomitant effect on developing mesenchymal structures, with modification or abolition of ectodermal–mesenchymal signaling. It is possible to approach this group of diseases basing on functional and molecular findings and to begin to explain the complex clinical consequences of mutations affecting specific developmental pathways. We have reviewed the molecular basis of ectodermal dysplasias applying this new clinical–functional classification. For each subset of the identified ED, we will now describe the genes and related proteins involved in terms of: (1) structure of the genes and their role in differentiation of the epidermis and the ectodermal derivatives; (2) genotype–phenotype correlation. © 2009 Wiley-Liss, Inc.