Autonomic dysfunction is common in critically ill patients, but it often remains unrecognized due to the elusive nature of the physiological alterations and their multifactorial influences. This review focuses on autonomic dysfunction encountered in patients with acute illness secondary to primary neurologic disorders and with non-neurologic acute systemic diseases. Our goal is to enhance the reader clinician's appreciation of autonomic disorders in a practical manner that enhances diagnostic understanding, anticipatory planning, and prognostication, in contrast to the usual phenomenological approach to the topic.