By analogy with ophthalmic migraine, hemiplegic migraine is defined by the occurrence during the attacks of unilateral weakness. This simple definition is however far from reflecting the wide range of clinical situations reported under this term. Familial hemiplegic migraine (FHM) is a well individualized autosomal dominant condition. Attacks start in childhood, adolescence, or early adulthood. They invariably include a unilateral weakness lasting 30 to 60 minutes and almost always associated with visual, sensory, or speech disturbances. They are occasionally very severe with a dense hemiplegia, confusion, coma or fever, but they always completely recover. Brain neuroimaging is normal. In 20% of the families, migraine is associated with permanent neurological signs, mainly nystagmus and cerebellar ataxia. FHM is a genetically heterogeneous condition, with half of the families linked to chromosome 19 and the other half in which this link is excluded. By contrast to FHM, which is a well defined entity, other varieties of so called hemiplegic migraine do not deserve to be individualized as such. They include attacks of migraine with typical aura when a unilateral weakness is part of the aura, severe hemiplegic attacks similar to those reported in FHM but sporadic, migrainous infarcts with hemiplegia, and, for some authors, alternating hemiplegia of childhood. The pathogenesis of all these conditions and of migraine itself remaining largely unknown, it is currently impossible to know whether or not they share common pathophysiologic mechanisms. The identification of the gene on chromosome 19 and the discovery of other genes will be major steps to elucidate this question.