Fever is caused by a resetting of the thermoregulatory center in the hypothalamus, most often by viral or bacterial infections. The specific etiologic diagnoses vary, depending on the child's age and height and duration of fever. Fever lasting 7 days or more is termed an FUO. When evaluating children who have fever, the most critical diagnostic maneuver is the carefully performed clinical evaluation, observation, history, and physical examination. Laboratory studies may be ordered based on the results of the clinical evaluation as well as the child's age and height and duration of the fever. Recent studies have outlined the manner in which data from the clinical evaluation and laboratory test results may be integrated to formulate an effective plan of management and follow-up.