Amyotrophic lateral sclerosis (abbreviated ALS, also referred to as Lou Gehrig's disease) is a form of motor neuron disease. ALS is a progressive, fatal, neurodegenerative disease caused by the degeneration of motor neurons, the nerve cells in the central nervous system that control voluntary muscle movement. The condition is often called Lou Gehrig's disease in North America, after the famous New York Yankees baseball player who was diagnosed with the disease in 1939. Today, renowned physicist Stephen Hawking is the best-known living ALS patient. The disorder causes muscle weakness and atrophy throughout the body as both the upper and lower motor neurons degenerate, ceasing to send messages to muscles. Unable to function, the muscles gradually weaken, develop fasciculations (twitches) because of denervation, and eventually atrophy because of that denervation. The patient may ultimately lose the ability to initiate and control all voluntary movement; bladder and bowel sphincters and the muscles responsible for eye movement are usually (but not always) spared.

Cognitive function is generally spared except in certain situations such as when ALS is associated with frontotemporal dementia. However, there are reports of more subtle cognitive changes of the frontotemporal type in many patients when detailed neuropsychological testing is employed. Sensory nerves and the autonomic nervous system, which controls functions like sweating, generally remain functional.