The motor neurone diseases are a group of disorders in which there is selective loss of function of upper and/or lower motor neurones in the motor cortex, brainstem, and spinal cord resulting in impairment in the nervous system control of voluntary movement. The term ‘motor neurone disease’, often abbreviated to ‘MND’, is used differently in different countries. In the United Kingdom it is used as an umbrella term to cover the related group of neurodegenerative disorders including amyotrophic lateral sclerosis, the commonest variant, as well as progressive muscular atrophy, primary lateral sclerosis, and progressive bulbar palsy. However, in many other countries amyotrophic lateral sclerosis, referred to as ALS, has been adopted as the umbrella term for this group of clinical variants of motor system degeneration. There is a tendency now internationally to use the ALS/MND abbreviation to cover this group of conditions. Careful diagnosis within the motor neurone diseases is essential for advising about prognosis, potential genetic implications, and for identifying those with acquired lower motor neurone syndromes who may benefit for the administration of immunomodulatory therapy.