Lissencephaly (smooth-brain) is an abnormality of brain development characterized by incomplete neuronal migration and a smooth cerebral surface. At least 2, and possibly more, distinct pathological types occur, each associated with several distinct syndromes. In this paper, the manifestations of 3 disorders associated with type I (classical) lissencephaly are discussed, including the Miller-Dieker syndrome with or without deficiency of 17p13, Norman-Roberts syndrome, and isolated lissencephaly sequence.