Electrodiagnostic methods play important roles for the diagnosis and evaluation of ALS. They are useful for 1) the early establishment of the diagnosis, 2) the differential diagnosis, and 3) the quantitative evaluation of the progression. Needle electromyography reveals active neurogenic changes over the wide territories of the body. Fibrillation potentials and positive sharp waves indicate presence of denervated fibers. However, they are not specific for ALS or even neurogenic process, and widespread occurrence of fibrillations are also characteristic for myositis and inclusion body myositis (IBM). Fasciculation potentials are spontaneous firing of the lower motor neuron and most of them are supposed to arise from the nerve endings. Fasciculation potentials are seen solely in neurogenic process and sufficiently specific for ALS. Its diagnostic role, especially in the early diagnosis, has been stressed by several researchers, including the present authors (Sonoo 1996), and fasciculation potentials will be given the same significance as fibrillation potentials in the coming Awaji criteria for the diagnosis of ALS. Motor unit potentials (MUPs) in ALS often show polyphasia and instability reflecting the presence of immature sprouts. Unstable MUPs (increased jiggle) are counterparts of instability in SFEMG. Giant MUPs are frequent, but low amplitude MUPs may be also observed in rapidly progressing cases. Observation of the recruitment pattern is crucial for the differential diagnosis from myopathies. Nerve conduction studies are important for the exclusion of other diagnoses, especially multifocal motor neuropathy (MMN). Some degree of slowing and disappearance of F-waves can occur simply due to loss of motor units. The utility of Neurophsiological Index remains to be confirmed. Repetitive nerve stimulation often reveals decremental responses, whose presence supports the diagnosis of ALS. Decremental responses in ALS usually occur in wasted muscles with low CMAP amplitude, and may predict the speed of further deterioration of the CMAP.