Hemifacial spasms occur in a variety of neurologic disorders, usually accompanied by additional neurologic signs and symptoms of systemic or regional origin. Hemifacial spasms occurring as isolated neurologic or psychoneurologic phenomena can be classified by simple clinical and electrophysiological observations into three main categories: hemifacial spasms due to (1) cortical irritation, (2) involvement of the facial nucleus or nerve radix, and (3) psychogenic conversion reactions (habit spasms). Hemifacial spasms due to cortical irritation are uncommon, are accompanied by repetitive and single spike firing from various mimetic muscles and also by focal disturbances of cerebral-neuronal rhythmicity in the eeg. Hemifacial spasms of nuclear or radicular origin can be subclasfied as (1) crypto-genic facial spasm (infectious, degenerative, inferior "anlage"), (2) "reaction a distance" (lesions of peripheral branches of the facial nerve), and (3) postparalytic (following Bell''s palsy). All show characteristic emg patterns, but no dfinite eeg disturbances or cerebral rhythmicity. The site of greatest clinical disturbance does not always correspond with the site of maximal emg flasciculatory spike-firing. This bears a relationship to the "selective partial neurectomy" of the facial nerve as the neurosurgical treatment of choice. Hemifacial spasms due to psychogenic conversion reactions are twice as common as are hemifacial spasms of organic etiology. They affect one or both sides of the face and can be temporarily abolished by intravenous sodium amytal injections. Emg recordings show muscle contraction bursts identical to the ones seen in voluntary movements.