Professor Stephen Hawking suffers from amyotrophic lateral sclerosis (ALS), the most common form of motor neurone disease.
What causes it?
ALS, which accounts for more than 90% of all cases of motor neurone disease, causes nerve cells in the brain and spinal cord called motor neurones to degenerate and eventually die.
These cells play an essential role in passing messages to the muscles.
Without them the brain cannot control movement properly, and the muscles cease to work well.
Around one in 10 cases of ALS are thought to be due to genetics, but the trigger for the other 90% of cases remains a mystery.
It is estimated that 3,500 people in the UK have ALS.
What are the symptoms?
As more and more motor neurones are lost, the muscles - particularly in the limbs - begin to waste.
Early symptoms include tripping up when walking, or dropping things.
Twitching and "cramping" of the muscles is also common, especially in the hands and feet.
In the more advanced stages, people often have difficulty speaking, swallowing or breathing and experience paralysis.
Death is usually caused by a failure of the respiratory muscles.
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