Nerve conduction velocity in degenerative disease depends on which nerve fibres are affected. If the largest diameter ones are spared, velocity will be normal. Otherwise, there will be a decrease of up to 30 per cent. The localising value of nerve conduction studies in degenerative disease is limited though hardly required after focal trauma. Electromyography will usually also be needed. An abnormal finding will always be found if either the stimulating or recording electrode lies over a region of degeneration. Preganglionic pathology occurring in radiculopathies is associated with normal sensory conduction. The degree to which sensory nerve action potentials (SNAPs) are reduced in amplitude is a useful guide to the severity of the pathology within postganglionic fibres. These concepts are illustrated with simple but helpful diagrams.