The most potent biologic toxin, botulinum toxin (BTX), has become a powerful therapeutic tool in the treatment of a variety of neurologic, ophthalmic, and other disorders manifested by abnormal, excessive, or inappropriate muscle contractions. This review focuses on the use of BTX in the treatment of dystonia and other movement disorders. The therapeutic application of BTX, however, extends beyond movement disorders; chemodenervation with BTX has been found to ameliorate spasticity, rigidity, spastic bladder, achalasia, and even some cosmetic conditions. In addition to describing its therapeutic effects, this article also reviews recent advances in the understanding of the molecular and cellular mechanisms of BTX. Few therapeutic agents have been better understood in terms of their mechanism of action or have had greater impact on patients' functioning than BTX. BTX-A has been used in nearly all clinical trials. Blocking anti-BTX-A antibodies have been detected in about 5% of patients chronically treated with this type of BTX. Patients who develop immunoresistance to BTX-A may benefit from other serotypes of BTX, such as BTX-B and -F, currently undergoing clinical trials.