Botulinum toxin type A, a neurotoxin, is effective for treating a variety of disorders of involuntary muscle contraction including cervical dystonia, blepharospasm, and hemifacial spasm. It inhibits neuromuscular signaling by blocking the release of acetylcholine at the neuromuscular junction. The biological effects of the toxin are transient, with normal neuronal signaling returning within approximately 3 to 6 months postinjection. Recent clinical findings suggest that botulinum toxin type A may inhibit pain associated with migraine and other types of headache. However, the mechanism by which this toxin inhibits pain is not fully understood and is under investigation. Research findings suggest that botulinum toxin type A inhibits the release of neurotransmitters from nociceptive nerve terminals and, in this way, may possess an analgesic effect. A number of retrospective open-label chart reviews and 3 double-blind, placebo-controlled trials have demonstrated that localized injections of botulinum toxin type A significantly reduce the frequency, severity, and disability associated with migraine headaches. Although the majority of patients in these studies experienced no botulinum toxin type A-mediated side effects, a small percentage of patients did report transient minor side effects including blepharoptosis, diplopia, and injection-site weakness. Currently, 4 randomized, placebo-controlled, clinical trials are being conducted to evaluate the efficacy, optimal dosing, and side-effect profile of botulinum toxin type A as a novel treatment for migraine and other types of headache. These studies may provide further evidence that botulinum toxin type A is an effective option for the preventive treatment of migraine.