Nosocomial pneumonia is the most important infectious disease in patients who require prolonged mechanical ventilation. Understanding of the etiology helps to prevent ventilator-associated pneumonia (VAP). VAP can develop in four ways: by aspiration, inhalation, hematogenous spread and by contiguous spread. The two most common are aspiration from the oropharyngeal region and inhalation, usually from manipulation of tubing or infected equipment. VAP is prevented by hand-washing, keeping the head of the bed at 45 and, in some cases, by treating the surface bacteria which usually cause VAP. Sputum can be used for the diagnosis of VAP in most of these patients instead of invasive bronchoscopy. However, if the patients are critically ill, then bronchoscopy is used. Treatment in these patients depends on the bacteria. Pseudomonas is treated by two drugs (beta-lactam plus a quinolone or aminoglycoside), Acinetobacteria by ampicillin/sulbactam or carbapenam, extended-spectrum beta-lactam-producing bacteria by carbapenums, and Staphylococcus by vancomycin or linezolid.