The spinal cord has a characteristic structure and functions that are distinct from those of the brain. Its functions are tremendously important since it modulates the peripheral sensory inputs to the dorsal horn, and it gives rise to the ascending pathways transmitting peripheral afferent inputs to the brain, and conveys the descending pathways from the brain both to the lower motor neurons, the final common pathway, and to dorsal horn sensory neurons. In spite of these vital functions, the spinal cord constitutes only a small percent­ age of the mass of the human central nervous system and is located far from the skin surface, which has obstructed the recording of its electrical activity. Recently, however, important advances have been made in several recording techniques, including epidural recording or averaging methods, allowing both sensory and motor evoked spinal cord potentials in man to be recorded. This volume is based on the papers presented at the Fourth International Symposium on Spinal Cord Monitoring and Electrodiagnosis. Each of these international symposia has brought together many of the specialists involved in this research, with an important increase in the number of participants since the first symposium was held in Toyko in 1981. At the past symposia several attempts were made to standardize data, techniques, and clinical applications and to integrate the new findings into patient care.