Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease, resulting in a damage of motor neurons. Pathogenesis of ALS is most probably multifactorial, complex and not entirely elucidated. It may involve such mechanisms as oxidative stress, toxicity of glutamic acid, dysfunction of the mitochondria, stress of endoplasmic reticulum, aggregation of proteins, dysfunction of the cytoskeleton, disturbed axonal transport, role of glial cells, neuroinflammatory process, lactic acid dyscrasia, and genetic factors. An important cause of oxidative stress in ALS are mutations of superoxide dismutase 1 (SOD1) gene leading to altered activity of the enzyme and its enhanced toxicity. Abnormal SOD1 participates in inflammatory response of activated astrocytes and microglia in the spinal cord of ALS patients. Mechanisms of oxidative stress and glutamate toxicity are coupled together. Death of motor neurons occurs as a result if activation of caspases and apoptosis, while mitochondrial dysfunction merely participates in the process. Pathomorphological alterations within the endoplasmic reticulum are present already at an early phase of the disease and indicate that stress within this structure plays an important role in the ALS-related process of neurodegeneration. Another interesting feature of ALS are alterations of cytoskeleton, concerning mainly neurofilaments. According to the hypothesis of lactic acid dyscrasia, dysregulation of myoneuronal lactic acid channel results in cellular stress, toxicity and progressive degeneration. An important role in the pathogenesis of ALS may be also played by genetic mutations of proteins other than SOD1.