Sudden and brief involuntary movements of central nervous system (CNS) origin called myoclonus may be cortical (motor strip), thalamocortical (thalamocortical loop) or reticular (caudal reticular formation). Epileptic, cortical and thalamocortical myoclonus are combined with a spike which, when it is focal, needs back-averaging to be demonstrated. Negative myoclonus due to lapse of tone can only be demonstrated during antigravidic posture and may be combined with either a slow wave or the second, positive component of a polyspike-wave. Epileptic myoclonus must be distinguished from epileptic spasms and tonic seizures, and from non-epileptic myoclonus, tics, tremor and chorea. Myoclonus may occur in partial symptomatic (mainly Rasmussen and dysplasia), cryptogenic (frontal) or idiopathic (negative myoclonus in CSWS) epilepsy. Generalized myoclonus is part of inborn errors of metabolism, non-progressive encephalopathy (mainly Angelman) and idiopathic epilepsy (juvenile and infantile benign and severe forms, and myoclonic-astatic epilepsy). Carbamazepine, vigabatrin and eventually lamotrigine may worsen myoclonus whereas it may be improved by benzodiazepines, valproate, lamotrigine, zonisamide and piracetam according to etiology. Pathophysiology must take in account maturation processes, lesions and genetic predisposition. However, precise mechanisms remain unknown and only hypotheses can be proposed, that could clarify the age-related EEG and clinical expression of the various syndromes.