Electromyography is used to differentiate primary muscle disease (myopathy) from abnormalities within the muscle resulting from pathology of its nerve supply (neuropathy). The diagnosis rests on the assessment of motor unit size and an estimate of motor unit numbers during voluntary activity, the recruitment pattern. That motor unit size is reflected in motor unit potential duration is explained and illustrated. Excessive polyphasic motor unit potentials are abnormal but do not, by themselves, distinguish between neuropathy and myopathy. The limitations of assessing motor unit numbers are acknowledged. Potentials may also be recorded from resting muscles. These may be normal as in end-plate potentials or end-plate noise, or abnormal as in fibrillation potentials, positive sharp waves, fasciculation potentials, myotonia, myokymia or complex repetitive discharges. Elegant examples of all of these are included.