Measurement of nerve excitability by threshold tracking provides complementary information to conventional nerve conduction studies and may be used to infer the activity of a variety of ion channels, energy-dependent pumps, and ion exchange processes activated during the process of impulse conduction. This review highlights recent clinical excitability studies that have suggested mechanisms for nerve involvement in a range of metabolic and toxic neuropathies. While clinical nerve excitability studies are still in their infancy, and it is too early to know whether they have diagnostic value, there is growing evidence of their utility to provide novel insights into the pathophysiological mechanisms involved in a variety of neuropathic disturbances.