Hospital-acquired pneumonia is the second most common nosocomial infection in the United States. Aspiration appears to be the major route for the entry of microorganisms into the lower respiratory tract. Nosocomial pneumonia may be caused by bacteria, viruses and fungi. Aerobic gram-negative bacilli and Staphylococcus aureus are the most common etiologic agents, but infection is usually polymicrobial. Risk factors for nosocomial pneumonia include host variables, colonization with nosocomial pathogens, and impaired response of pulmonary defenses to the microbial challenge. Bacteria causing nosocomial pneumonia may be part of the patient's endogenous flora, originate from the hands of hospital personnel, or result from the use of invasive devices. The mechanically ventilated patient has multiple risk factors that contribute to the high rate of nosocomial pneumonia. An understanding of the epidemiology and risk factors for nosocomial pneumonia is fundamental for implementation of preventive strategies to reduce patient morbidity, mortality, and hospital costs.