Microcephaly is defined as a reduction in head circumference and this clinical finding infers that an individual has a significant diminution in brain volume. Microcephaly can be usefully divided into primary microcephaly, in which the brain fails to grow to the correct size during pregnancy, and secondary microcephaly, in which the brain is the expected size at birth but subsequently fails to grow normally. Current work suggests that primary microcephaly is caused by a decrease in the number of neurones generated during neurogenesis, but that in secondary microcephaly it is the number of dendritic processes and synaptic connections that is reduced. Important insights into human neurogenesis are being revealed by the study of rare genetic diseases that involve primary microcephaly, illustrated by the identification of the Microcephalin, abnormal spindle in microcephaly and ataxia-telangiectasia and Rad3-related genes. Furthermore, these findings facilitate the search for the evolutionary changes that have lead to the human brain being so much larger than that of any other primates.