Human microcephaly comprises a heterogeneous group of conditions that are characterized by a failure of normal brain growth. Microcephaly can be caused by many injurious or degenerative conditions, or by developmental malformations in which the growth of the brain is impaired as a result of defects in pattern formation, cell proliferation, cell survival, cell differentiation, or cell growth. These latter forms of congenital microcephaly are frequently inherited, usually as recessive traits, and are associated with mental retardation and sometimes epilepsy. Some of the genes that cause congenital microcephaly are likely to control crucial aspects of neural development, and may also be involved in the evolutionary explosion of cortical size that characterizes primates. There has recently been a rapid advance in the use of genetic mapping techniques to identify genetic loci responsible for microcephaly. Although several loci have been mapped, the condition is clearly genetically and clinically heterogeneous.