This chapter presents the history, clinical findings, pathology, differential diagnosis and management of propriospinal myoclonus. Propriospinal myoclonus at sleep onset consists of sudden axial jerks arousing during drowsiness preceding falling asleep and during intra-sleep relaxed wakefulness and upon awakening in the morning. The jerks may be variable in frequency, recurring in clusters separated by long gaps of time or quasi-periodically every 10-20 sec during transition from wakefulness to sleep. For patients with suspected propriospinal myoclonus, multiple muscle EMGs from cranial, axial and limb muscles are essentials at polysomnographic investigation. According to the evidence available, propriospinal myoclonus at sleep onset must originate at the spinal level, but it must be set into motion by neurophysiological mechanisms specific to the transitional period between wake and sleep. Propriospinal myoclonus at wake-sleep transitions remains idiopathic condition, often lifelong one, in which no structural abnormality of the brain or spinal cord has been demonstrated.