Myoclonus is complex, and is likely to encompass a number of relatively disparate phenomena. A widely used approach to the classification of myoclonus is a physiological one, in which the major forms are cortical and subcortical. In this classification, cortical forms of myoclonus are defined by the presence of enlarged somatosensory evoked potentials (SEP), back-averaged potentials, and enhanced long latency reflexes, whereas subcortical forms are largely delineated by the absence of these features. In addition to cortical and subcortical types, the presence of generalized spike and wave discharges on EEG indicate the presence of cortical-subcortical interactions, analogous to absence seizures of idiopathic generalized epilepsy. However, a number of difficulties arise in applying these criteria to many forms of myoclonus: 1. With regard to enlarged evoked potentials, double dissociations may be found when contrasting disease entities which are known to affect cortical or subcortical structures. 2. On back-averaging, the latency of the pre-myoclonic potential may be incompatible with a primarily cortical origin for the myoclonic jerk. 3. The triad of enhanced SEP, back-averaged potential and long latency reflex may all be present or only one component may be present, suggesting different pathophysiological mechanisms underpin these phenomena. One can conclude that it is likely that many forms of myoclonus lie on a spectrum between myoclonus and tremor. Some have clearcut evidence for being of cortical origin, whereas others are cortical-subcortical or purely subcortical. The role of subcortical structures in influencing electrophysiological phenomena is poorly understood, and merits further investigation.