Botulinum toxin has been shown to effectively treat several types of pain associated with neurologic disorders. It has recently been evaluated for the treatment of various types of headaches. In studies of migraine headache, chronic daily headache (more than 15 days of headache per month), tension-type headache, and post-whiplash headache, patients have reported decreased pain after treatment with botulinum toxin type A. A more recently available preparation, botulinum toxin type B, has also been shown to provide relief to patients with transformed migraine headache and post-whiplash headache. Additional study is under way to determine the effectiveness of botulinum toxin for the treatment of chronic cluster headaches. Although the pathophysiology of headache is incompletely understood, muscle tension may trigger or aggravate migraine headaches. Botulinum toxin, which reduces muscle hyperactivity, may reduce headache pain by decreasing muscle tension. It may also provide peripheral and central neurogenic effects and reduce inflammation. Large, rigorously controlled trials of botulinum toxin are needed to better characterize its effects on various types of headaches and its role as a therapeutic agent. Current data suggest that botulinum toxin is safe and does not produce systemic effects associated with other types of headache treatments.