Nosocomial pneumonias are due to various causes and their development depends mainly on the underlying condition of the patients. Intubated patients are prone to develop bacterial pneumonia, with microorganisms found in the oro-pharyngeal or gastric flora. Prevention relies on diminishing the exogenous and endogenous colonization of the tracheobronchial tree through: avoidance of cross contamination, maintenance of a physiological gastric pH and, possibly, selective digestive decontamination.