Neurologic complications occur in over half of all critically ill patients and may affect outcome as well as length of stay. The most common complications are encephalopathy, seizures, stroke, peripheral neuropathy, and hypoxic-ischemic damage. The principal critical illness that is associated with most of these complications is sepsis, but the specific pathobiology within the nervous system is not well known. Transplantation accounts for the main other critical illness, and complications of hepatic failure and transplantation are discussed. Use of magnetic resonance imaging in comatose patients undergoing transplantation may be revealing, whereas computed tomographic images are not. Diagnosis and monitoring of the nervous system in the intensive care unit in order to detect and manage these complications remains the challenge for the future.