Postoperative fever should be evaluated with a focused approach rather than in "shotgun" fashion. Most fevers that develop within the first 48 hours after surgery are benign and self-limiting. However, it is critical that physicians who provide postoperative care be able to recognize the minority of fevers that demand immediate attention, based on the patient's history, a targeted physical examination, and further studies if appropriate. Fever that develops after the first 2 days following surgery is more likely to have an infectious cause, but noninfectious causes that require further evaluation and treatment must also be considered. When evaluating postoperative fever, a helpful mnemonic is the "four Ws": wind (pulmonary causes: pneumonia, aspiration, and pulmonary embolism, but not atelectasis), water (urinary tract infection), wound (surgical site infection), "what did we do?" (iatrogenic causes: drug fever, blood product reaction, infections related to intravenous lines).