Hemifacial spasm (HFS) is a peripherally induced movement disorder causing clonic or tonic contractions of the facial muscles. HFS is usually unilateral and sporadic. It may be primary (mainly attributed to vascular compressions of the seventh cranial nerve in the posterior fossa) or secondary to facial nerve or brainstem damage. The two forms share a number of features but may differ in clinical presentation (simultaneous involvement of the upper and lower facial muscles in secondary forms). The spasm-related electromyogram activity is probably generated by ephaptic transmission, due to local demyelination at the entry zone of the facial nerve root (possibly owing to nerve damage caused by a compressing cerebral vessel). These findings suggest the "nerve origin hypothesis" as the main pathophysiological mechanism underlying HFS. Medical treatment (anticonvulsants or GABAergic drugs) is generally ineffective. Microvascular decompression of the facial nerve can achieve marked improvements in the majority of patients, although recurrences and complications are not uncommon. Local (orbicularis oculi or lower facial muscles) injection of Botulinum toxin (BoNT) is therefore considered the preferred symptomatic treatment for primary HFS. The long-term efficacy and safety of BoNT have been documented by clinical studies.