The ectodermal dysplasias are a heterogeneous group of disorders that, in the past, has included conditions best classified as progeroid disorders. The inaccuracy of the terminology has led to a proliferation of syndromes in which the patients are said to have poorly defined "ectodermal dysplasia," and a real need exists to define that appellation further. We suggest that the term "ectodermal dysplasia" be limited to those disorders that are congenital, are diffusely present, are not progressive, and do involve the epidermis and at least one of the appendages. We recognize that a heterogeneous group of disorders remains that generically have certain similarities. Not enough is known about the defects in each of the elements of the skin affected in these conditions to classify them more accurately. Several recently described disorders appear to have some degree of ectodermal dysplasia.