Hospital-acquired pneumonia in the postoperative or trauma patient is a common and troublesome complication. A variety of patient factors such as smoking, chronic obstructive pulmonary disease (COPD), morbid obesity, and poor overall medical condition, along with therapeutic interventions such as abdominal and thoracic operations, the need for prolonged intubation and mechanical ventilation, and postoperative changes in respiratory mechanics increase the risk for nosocomial pneumonia in surgical patients. Establishing the diagnosis of hospital-acquired pneumonia in this group of patients is notoriously difficult, since systemic signs of inflammation and radiographic changes of atelectasis or chest trauma that mimic infectious infiltrates are common. Early-onset and late-onset pneumonia in these patients have different associated risk factors, microbiology, and prognosis. Late-onset pneumonia in both postoperative and trauma patients is associated with significant mortality. A variety of preventive measures such as selective digestive decontamination, continuous postural oscillation, and preoperative respiratory muscle training may be beneficial in selected circumstances.