This chapter talks about the neurocutaneous syndromes such as hypomelanosis of Ito (HI), incontinentia pigmenti (IP), nevus sebaceous (NS) syndrome and unilateral somatic intracranial hypoplasia. Chromosomal mosaicism is recognized as the pathogenic basis of many cases of HI and related disorders. It can explain the protean clinical manifestations of this condition and their often asymmetrical expression. Cerebral lesions of IP patients commonly extend radially through cortical and subcortical zones, involving cortex, subcortical and deep white matter, ependymal and subependymal zones of one or both cerebral hemispheres. Epilepsy usually appears after a variable period of evolution when the subcortical lesions are apparent in the cerebral hemisphere ipsilateral to the facial hemiatrophy. Unilateral hypoplasia of a polymicrogyric cerebral hemisphere, of the brainstem, cerebellum, and of the intracranial arteries on the same side and a hypoplastic hemibody commonly occur.