In the ICU, fever can be expected to accompany an extensive number of conditions of both infectious and noninfectious etiologies. It is crucial to identify the precise cause of fever, because certain conditions in either category may be life-threatening, whereas others require no treatment at all. It is important to rule out the most common infections that may be present based on historical and physical signs and symptoms and epidemiologic factors. The extent of evaluation should be based on the likelihood of the disease process being present and is highly variable for each individual patient. Therefore, "routine fever work-up" should not be advocated. If overt infection is not found upon initial evaluation, antibiotics should be withheld if possible. Alternatively, in the unstable patient, empiric therapy may be started, and if no infection is evident, it may be stopped within a reasonable time frame. In no case should prolonged antibiotics be given for presumed but unproven infection. Thorough knowledge of the more common infectious and noninfectious conditions, as well as the awareness of less frequent ones and their predisposing risk factors, is essential for adequate evaluation of the febrile ICU patient. Likewise, familiarity with the techniques used for diagnosis of these infections and their appropriate interpretation and limitations in specific instances is immensely helpful to the clinician providing appropriate care for the critically ill patient.