Amyotrophic lateral sclerosis (ALS) is among the most dire neurological diseases. The essential clinical feature of the disorder is relentless, lethal paralysis, usually beginning in midadult years. The disease is caused by a slow, progressive loss of motor neurons in the brain and spinal cord. It usually begins focally and then spreads. In most cases, there is concurrent involvement of corticospinal (upper) and spinal (lower) motor neurons, although in some instances the spinal motor neuron features predominate. Involvement of the spinal motor neurons produces muscle denervation of the affected muscles and fasciculations, followed by muscle atrophy. When corticospinal motor neurons degenerate, the weakness is accompanied by spasticity. The mean age at onset of ALS is 55 years; the mean duration is about 4 years. The incidence of new cases is approximately 1 per 100,000 population. The total number of cases is about 5 per 100,000 population. In the United States, it is estimated that there are 20,000 to 30,000 cases. About 10% of cases are inherited as an autosomal dominant trait; familial and sporadic ALS are clinically indistinguishable.