Genetic microcephaly and lissencephaly are 2 of the most common brain malformations. Each of them is a heterogeneous group of disorders caused by mutations of many different genes. They are a significant cause of neurological morbidity in children worldwide, responsible for many cases of mental retardation, cerebral palsy, and epilepsy. Recent advances in molecular genetics have led to the identification of several genes causing these disorders, and thus accurate molecular diagnosis and improved genetic counseling has become available for many patients and their families. More recently identified genes include STIL, causing primary autosomal recessive microcephaly (microcephaly vera), and TUBA1A, causing lissencephaly. Numerous other disease genes are likely still to be identified. Functional studies of genes that cause microcephaly and lissencephaly have provided valuable insight into the molecular mechanisms of human brain development.