Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by the degeneration of upper motor neurons in the brainstem and spinal cord and lower motor neurons. ALS was first described by Jean-Martin Charcot in 1874 based on clinical features and postmortem examinations. In 1990, the first diagnostic criteria for ALS were developed based on clinical features. Subsequently, three additional diagnostic criteria were published. In this article, we introduce the clinical features, diagnostic criteria, and diseases that need to be differentiated in ALS.