Disorders of elevated body temperature may be classified as either fever or hyperthermia. Fever is caused by a pyrogen-mediated upward adjustment of the hypothalamic thermostat; hyperthermia results from a loss of physiologic control of temperature regulation. Fever in the ICU can be due to infectious or noninfectious causes. The initial approach to a febrile, critically ill patient should involve a thoughtful review of the clinical data to elicit the likely source of fever prior to the ordering of cultures, imaging studies, and broad-spectrum antibiotics. Both high fever and prolonged fever have been associated with increased mortality; however, a causal role for fever as a mediator of adverse outcomes during non-neurologic critical illness has not been established. Outside the realm of acute brain injury, the practice of treating fever remains controversial. To generate high-quality, evidence-based guidelines for the management of fever, large, prospective, multicenter trials are needed.