The motor neuron diseases (MNDs) are a family of diseases commonly categorized by their propensity to affect upper or lower motor neurons and by their mode of inheritance. The chapter provides some content on infectious MNDs caused by viral infections affecting the motor neurons in the anterior horn of the spinal cord. However, the chapter devotes most of its attention to the inherited and sporadically occurring MNDs. The majority of research into adult MND focuses on amyotrophic lateral sclerosis (ALS) due to its high prevalence, rapid progression, and phenotypical similarities between its inherited form and its sporadic form. As our knowledge of genetic mechanisms underlying ALS pathology has grown, common themes have emerged. These include abnormalities in RNA biology, axonal transport, protein folding, and inflammatory responses. These themes currently drive much of the direction in ALS experimental therapy development. It is clear that MND is complex and involves several different molecular pathways. Given this complexity, ALS might not be a single disease entity, and if this is the case, treatment approaches may need to be targeted to specific pathologies rather than all ALS patients on a broad scale. Chapter content is enhanced by tables outlining the types of MNDs, criteria for supporting a diagnosis, first-line workup, the genes associated with ALS, ALS efficacy outcome measures, symptom management of ALS, and spinal muscular atrophy classification. Mechanisms of ALS are illustrated, and clinical photographs demonstrate symptoms. This review contains 5 figures, 14 tables and 253 references Keywords: motor neuron disease, amyotrophic lateral sclerosis, electromyography, Guillain-Barré syndrome