Electrophysiology is important in amyotrophic lateral sclerosis (ALS). It helps in the assessment of disease severity and rate of progression, and it plays a role in evaluating the efficacy of therapeutic trials. Presently, electrophysiology is the only means of confirming suspected ALS, and this has been incorporated into the El Escorial criteria. Needle electromyography identifies disease in clinically "unaffected muscles" including bulbar musculature, confirms involvement of anterior horn cells, and can detect early involvement of respiratory muscles (intercostals and diaphragm). Conduction studies are imperative to rule out motor neuropathy with multifocal conduction block. Various techniques (cortical threshold, cortical silent period, double stimulation) employing transcranial magnetic stimulation have demonstrated that the motor cortex in ALS is hyperexcitable. Central motor conduction is normal in ALS but uniquely slow in the D9OA SOD1 mutation. Using peristimulus time histograms (PSTHs) it is possible to estimate the size of a unitary excitatory postsynaptic potential (EPSP). In ALS the EPSP is typically desynchronized. With time it becomes reduced in amplitude and a slow conducting component becomes recognizable in the primary peak of the PSTH. This reflects conduction through a slow motor pathway. Abnormalities of the PSTH are not seen in Kennedy's disease, implying that the changes seen in ALS are due to supraspinal mechanisms.