Studies of the excitability of human axons using threshold tracking can provide insight into peripheral nerve dysfunction, complementing routine tests of nerve conduction (Kiernan et al. 2000). The latter measure the ability of axons to conduct and their speed of conduction; the former provide information about the voltage-dependent conductances, pumps and other mechanisms responsible for maintaining membrane potential at the site of stimulation. The reliability of conclusions from these studies has been improved by the development of a computer model of the human motor axon (Bostock, 2006). Threshold tracking relies on measuring the current required to produce a sub-maximal compound potential of usually ~40% of maximum (because that is on the fast rising phase of the stimulus-response curve). In normal subjects the testing protocol measures a stimulus-response curve, strength-duration properties, the threshold changes produced by subthreshold depolarizing and hyperpolarizing currents (threshold electrotonus and the currentthreshold relationship), and the recovery of excitability following axonal discharge, all in about 10min. Abnormalities have been defined in many peripheral nerve disorders. In addition, peripheral nerve function may be abnormal in primarily central disorders (GEFS+; episodic ataxia) and when there is central damage (stroke, Jankelowitz et al. 2007; MS, Ng et al. 2008; spinal cord injury, Lin et al. 2007). Examples of the latter will be shown.