In this article the principles and practice of clinical EMG are described. The basic components of EMG instrumentation include specialized intramuscular recording electrodes, a preamplifier, amplifier, and displays. Displays are usually both visual and auditory, using the CRO and a loudspeaker, respectively. The motor unit is the functional unit of muscles and is the anatomic basis for clinical EMG. There are distinct sites along the motor unit pathway where pathologic changes may produce EMG abnormalities. These sites include (1) anterior horn cell; (2) spinal nerve root; (3) plexus; (4) peripheral nerve; (5) myoneural junction; and (6) muscle fiber. Normal EMG potentials in resting muscle (which is predominantly silent) include end-plate potentials and miniature end-plate potentials, which are present only in the region of the motor end plate. Individual motor unit potentials can be observed when a muscle contracts minimally. The morphology of motor unit potentials varies within a normal range, which is somewhat specific for each muscle, depending on its nerve-muscle fiber innervation ratio. Stronger contraction of a muscle produces an orderly recruitment of motor units, referred to as an interference pattern. In resting muscle the most commonly encountered abnormal potentials include (1) positive sharp waves, (2) fibrillation potentials, (3) fasciculation potentials, and (4) high frequency discharges. Abnormalities in motor unit morphology can be detected best in minimally contracting muscles. Polyphasic motor units contain more than four phases and constitute less than 15 per cent of all motor units in a given muscle. In myopathy the motor unit potentials are often polyphasic. They are of low amplitude and short duration. In neuropathy motor unit potentials may also be polyphasic; however, the size of the motor unit is either normal or of increased amplitude and duration depending on chronicity. Such findings in myopathy and neuropathy correlate with known pathoanatomic changes in these conditions. In myopathy the motor unit interference pattern will often be normal or enhanced despite clinical weakness in the muscle. In neuropathy the interference pattern will be reduced, and when neuropathy is severe a single large motor unit may produce a single motor unit pattern. Besides its application as a valuable aid in diagnosis of neuromuscular disorders, electromyography is also utilized for prognosis, determining the need for surgery, planning programs of rehabilitation, and providing evidence for medical legal purposes. Electromyographic findings most often serve as an adjunct to a thorough clinical evaluation of the patient. The electrophysiological data obtained may help support or rule out a specific clinical diagnosis.