Progressive loss of motor neurons causes Amyotrophic Lateral Sclerosis. Patients complain, most often, of progressive weakness in the distal limbs. However, weakness may manifest in any body segment (bulbar, cervical, thoracic, or lumbosacral). The diagnosis of ALS is suggested by clinical examination that reveals both upper and lower motor neuron failure. Formal diagnostic criteria have been developed and validated. Nerve conduction and electromyography studies improve diagnostic sensitivity and exclude some alternate, treatable diagnoses. Likewise, conventional imaging studies and laboratory evaluation refute other diseases that may masquerade as ALS. Experimental imaging and laboratory evaluations may improve ALS diagnosis in the future. The cause of motor neuron death is not known but inherited forms of motor neuron disease may suggest mechanisms. The goal of ALS treatment is control of the symptoms of progressive weakness, especially respiratory insufficiency and dysphagia and is best managed in an integrated clinic.