Hemiplegic migraine occurs both sporadically and as a familial syndrome. Familial hemiplegic migraine is characterized by migraine with aura and motor weakness and at least 1 first- or second-degree relative with migraine aura and motor weakness. Familial hemiplegic migraine is separated into FHM-1, FHM-2, FHM-3, and other loci depending on specific genetic mutations. Sporadic hemiplegic migraine has the same clinical features as familial hemiplegic migraine, but no family history of motor weakness. However, familial hemiplegic migraine mutations have been found in some sporadic hemiplegic migraine patients. PRRT-2 mutations have been identified in some hemiplegic migraine patients. New mechanisms underlying cortical spreading depression and cortical hyperexcitability in hemiplegic migraine patients have been proposed. Evidence suggests that familial hemiplegic migraine pathophysiology is different from common forms of migraine.