West syndrome is a multi-etiological condition. Recent progress in perinatal medicine and the recent development of new neuroimaging techniques may have changed the etiological panorama of West syndrome. Our recent study has disclosed an increasing percentage of the perinatal group and a decreasing percentage of the doubtful group. The increase of the perinatal group is due to an increased proportion of very low-birthweight infants and periventricular leucomalacia (PVL). Among various etiological factors added to the long list of causes of West syndrome, focal cortical dysplasia is another newly emerging etiological factor associated with this syndrome. Patients with unilateral focal dysplasia more commonly have partial seizures, but may show infantile spasms transiently during infancy. They may have partial seizures preceding, in combination with or following infantile spasms. Follow-up MRI is necessary to detect delayed myelination because it is not disclosed at common ages of onset of this syndrome. PET is useful to further differentiate the cryptogenic group. Although West syndrome is regarded as one of the intractable epilepsies, the prognosis differs widely according to etiology. Follow-up PET is also useful to predict seizure and psychomotor prognosis.