The phakomatoses appear to represent a group of disorders characterized, in part, by the common features of neuroectodermal mal-development, undecided differentiation of cells, and disturbed patterns of cell migration. With progress in genetics and cell biology, the precise deficits in the genome and the specific biochemical abnormalities associated with these defects will be elucidated. It is expected that the apparent diversity of these lesions will then be understood as a coherent series of missteps along the common pathway of normal growth and development.