A motor neuron disease (MND) is any of five neurological disorders that selectively affect motor neurons, the cells that control voluntary muscles of the body. These five conditions are amyotrophic lateral sclerosis, primary lateral sclerosis, progressive muscular atrophy, progressive bulbar palsy and pseudobulbar palsy. They are neurodegenerative in nature and cause increasing disability and, eventually, death.

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease and Charcot disease, is a specific disorder that involves the death of neurons. In a number of countries the term motor neurone disease (MND) is commonly used, while others use that term for a group of five conditions of which ALS is the most common. ALS is characterized by stiff muscles, muscle twitching, and gradually worsening weakness due to muscles decreasing in size. This results in difficulty speaking, swallowing, and eventually breathing.