In the early days, amyotrophic lateral sclerosis (ALS) had the same meaning as motor neuron disease, specifically referring to an independent disease in which lower motor neurons were damaged first and then upper motor neurons were damaged. But later it was discovered that there were two other variants, namely, only upper motor neurons or lower motor neurons were always involved in the course of the disease. The former was called primary lateral sclerosis and the latter was called spinal muscular atrophy. So far, some literature still uses motor neuron disease to specifically refer to amyotrophic lateral sclerosis. Most scholars are accustomed to dividing motor neuron disease into three types: amyotrophic lateral sclerosis, primary lateral sclerosis and spinal muscular atrophy based on different combinations of upper and lower motor neuron involvement. Recent studies have suggested that amyotrophic lateral sclerosis has a common pathological basis with a variety of related diseases, including primary lateral sclerosis, ALS-dementia, ALS-related frontal lobe dementia, progressive spinal muscular atrophy, multiple system atrophy and Lewy body disease. Pathological examination revealed that these diseases also contained ubiquitin-positive inclusions and hyaline mass inclusions, but they damaged different anatomical sites and presented various clinical combinations.