A comprehensive approach to diagnosis of patients with lissencephaly using clinical, CT and MRI scan, and sometimes other laboratory data will allow specific diagnosis to be made in a large majority of patients. The most common diagnoses in order of decreasing frequency will probably prove to be WWS, MDS, and ILS. The remainder constitute a heterogeneous group. Both diagnosis and counseling have been modified by several recent and important advances. Diagnostic criteria for WWS have been revised. Several molecular probes have been located within the MDS critical region in chromosome band 17p13.3. Prenatal diagnosis should prove to be reliable in both WWS and MDS.