Electrodiagnostic testing has proved useful in helping to establish the diagnosis of amyotrophic lateral sclerosis by eliminating possible disease mimics and by demonstrating abnormalities in body areas that are clinically unaffected. Electrodiagnosis begins with an understanding of the clinical features of the disease, because clinical correlation is essential. To improve the sensitivity of the electrophysiologic evaluation, the Awaji criteria have been proposed as a modification to the revised El Escorial criteria. Although techniques to evaluate corticomotor neuron abnormalities and to quantify lower motor neuron loss have been developed, they remain primarily research techniques and have not yet influenced clinical practice.