Clinical electrophysiologic measures derive from sound neurophysiologic principles and provide sensitive, objective information useful in the evaluation of generalized polyneuropathy. The electrodiagnostic examination includes sensory and motor nerve conduction studies, evaluation of late responses, and needle electromyography. These measures represent an extension of the neurologic examination. Clinicians use electrodiagnostic information to confirm clinical findings, localize specific abnormalities to a degree not clinically possible, and identify the underlying pathophysiology. Although the primary role of clinical electromyography is diagnostic, test results are sufficiently sensitive to identify subclinical findings and to monitor small changes related to disease progression or treatment response. In some peripheral disorders, electrodiagnostic information provides the most sensitive indicator of prognosis. Classification of generalized polyneuropathy using electrophysiologic information focuses the differential diagnosis, directs the subsequent evaluation, and often suggests a specific diagnosis or class of disorders. Several of the most common treatable polyneuropathies were rarely diagnosed 25 years ago. Awareness of these disorders relates to increased utilization of clinical electrophysiology and identification of characteristic electrodiagnostic features that result in their recognition.