Sepsis and critical illness occur as complications of illness, injury, or surgery in approximately 5% of patients in our critical care unit. Clinical evaluation of the nervous system is difficult in this clinical setting, and electrophysiologic studies are therefore quite valuable. Electroencephalography detects encephalopathy and electromyography (EMG) and nerve conduction studies detect neuromuscular disorders at early stages of their development. Thus, septic encephalopathy occurs in almost all patients and critically ill polyneuropathy in at least 50% of such patients. The polyneuropathy is a predominantly distal axonal degeneration of motor and sensory fibers. A catabolic myopathy is also present, but is difficult to detect electrophysiologically. No defect in neuromuscular transmission has so far been demonstrated. Both the encephalopathy and polyneuropathy may be quite severe, but with vigorous management of the sepsis and critical illness complete recovery may occur in the 40% of patients who survive.