Infantile spasms (IS) is usually classified as a form of "myoclonic epilepsy," but the nosology of this whole group of disorders is unclear. Evidence suggests that the spasms are subcortically mediated, but can be modified by input from the cortex, which is believed to be abnormally excitable and disorganized. The latter features may give rise to hypsarrhythmia. The whole issue of myoclonus rests on the phenotype of IS and precise measurements of the length of electromyographic (EMG) bursts. Based on scant EMG data, it would appear that the bursts during flexor spasms are too long for epileptic myoclonus. The nature of tonic spasms of even longer duration is not myoclonic. However, the infrequent spontaneous myoclonic jerks, which can occur without spasms, and head nodding could represent positive and negative myoclonus, respectively. Data can be collected easily through techniques such as back-averaging to resolve the issue of classification and localization of motor phenomena.