Electrophysiology assesses function and should be considered complementary to, not as competing with, myelography, which assesses structure. Electromyography, being devoid of morbidity and significant side effects, should precede invasive radiology: myelography and contrast-enhanced computerized tomography (CT). Needle electromyography is the most useful electrophysiologic test when evaluating radiculopathies. Presence of denervation (fibrillation and positive sharp waves) indicates axonal interruption, a major determinant to the completeness and speed of recovery. It occurs first in the paraspinal muscles. In the first few days before denervation can develop, abnormalities of motor unit recruitment are most helpful in differentiating weakness due to impaired conduction as opposed to pain or voluntary lack of effort. An abnormal paraspinal EMG is electrophysiologic proof of a lesion at or proximal to the spinal root. Such abnormalities persist for long periods following surgery, limiting the use of EMG in the postoperative period. Conventional conduction studies are usually normal in radiculopathies. An abnormal or unrecordable sensory nerve action potential should suggest a "ganglionopathy" or double crush syndrome. F wave, H reflex, and somatosensory evoked potential studies, although having a lower diagnostic yield than needle EMG, can be helpful in confirming some root lesions early in the disease. The diagnostic value of these tests might be increased by consideration of characteristics other than latency such as their amplitude and dispersion.