Migraine is a common and debilitating disorder that often requires prophylactic therapy, particularly for those migraine patients who meet the diagnostic criteria for chronic daily headache (chronic migraine). Existing prophylactic treatments for migraine are inadequate for many patients due to their modest efficacy and/or systemic side effects. Alternative treatment strategies are needed, particularly in those with chronic migraine. Botulinum toxin type A is a locally injected protein complex that has been investigated as a treatment for episodic migraine and chronic daily headache. A systematic series of controlled trials has led to the identification of a subset of migraineurs with chronic daily headache who obtain demonstrated benefits of botulinum toxin type A over placebo that is maintained with repeated treatments.