Gray matter heterotopia is a set of neuronal migration disorders characterized by collections of nerve cells in abnormal locations secondary to arrest of the radial migration of neurons. These malformations can be divided into three groups according to their location: periventricular nodular heterotopia, focal subcortical heterotopia, and subcortical band heterotopia. The most frequent manifestation in patients with gray matter heterotopia is epilepsy, however, the epileptogenic mechanism has not been clarified yet. Several gene abnormalities responsible for each type of heterotopia have been investigated, and the major genes identified include FLNA (Xq28) and ARFGEF2 (20q13) in periventricular nodular heterotopia, and LIS1 (17p13.3) and DCX (Xq22.3-q23) in subcortical band heterotopia. The specific gene abnormalities associated with focal subcortical heterotopia remain unknown. Not only genetic factors, but also environmental factors, have been implicated in the etiology of neuronal migration disorders. Disruption of the radial glial fibers and the resultant migration arrest has been suggested to be an essential mechanism inducing gray matter heterotopia due to an environmental origin.