Electrodiagnostic studies play an important role in the evaluation of patients suspected of having a myopathic disorder. They are used to exclude alternative diagnoses, confirm the presence of muscle disease, narrow down the differential, and identify an appropriate biopsy site. The most informative part of the electrodiagnostic study is needle electromyography. This allows for the analysis of spontaneous activity and motor unit action potential morphology and recruitment patterns. This article proposes a practical electrodiagnostic approach and describes the electrophysiologic patterns of the most commonly encountered myopathies.