Microcephaly is defined as a small cranium with an occipito-frontal head circumference (OFC) of more than two standard deviations (SD) below the mean for age, sex, and ethnicity. Microcephaly can be congenital (primary microcephaly) or develop postnatally (secondary microcephaly). Either type can be caused by environmental or genetic factors. Approximately 15-50% of individuals with microcephaly have been reported to have an underlying genetic etiology. Individuals with primary microcephaly have inadequate brain growth during pregnancy, and are born with a significantly small head size (OFC of < 3 SDs). They may also develop mild to severe intellectual disabilities, seizures, mild short stature, and a narrow sloping forehead due to the reduced cranial size. Imaging typically reveals a normal brain structure with reduction in size, particularly of the cerebral cortex. The brain size in cases of secondary microcephaly has the expected size at birth but subsequently fails to grow normally. Common causes of secondary microcephaly include environmental insults, such as infections or a brain injury. In addition, secondary microcephaly is observed in some metabolic disorders or genetic syndromes, such as Rett syndrome and Angelman syndrome, in which a progressive reduction in head circumference is seen in infancy or early childhood.Congenital and postnatal microcephaly could present as an isolated finding in an individual, be associated with other brain malformations such as cerebellar hypoplasia, or as part of an underlying syndrome.