A macro- and/or microscopical study on the normal and abnormal development of the forebrain with the eyes, nose, and cranium, was performed in 139 mouse embryos, 120 normal and 19 abnormal human embryos and fetuses, and in about 2,300 human skulls. The results suggest that from the embryological point of view, a distinction should be made between facial defects involving the brain and/or the neural elements of the eyes, i.e., the cerebro-craniofacial dysplasias, and malformations of the face and cranium only, called the craniofacial dysplasias. Both groups can be subdivided into early or primary defects (in embryos less than or equal to 17 mm C-RL) and late or secondary defects (in embryos greater than or equal to 17 mm C-RL). Almost all of the primary defects can be considered to originate from disorders occurring during the transformation of the brain and face. The secondary defects concern defective differentiation of neurectoderm and of the mesenchyme into bone centers, cartilage, and muscles. All of the defects in question can be explained by insufficient cell proliferation, degeneration, and/or differentiation. New terminology is proposed.