The most common route of entry of pathogens into the lung in patients managed in the intensive-care unit is aspiration of contaminated oropharyngeal secretions or gastric contents. In intubated patients, the risk of this type of infection is particularly high. Knowledge of specific risk factors for specific microorganisms, along with the origin of acquisition (primary endogenous, secondary endogenous, or exogenous), will permit a more rational and effective method of prevention. Attributable mortality is highly dependent on the institution of a correct initial antibiotic choice, as well as the interaction between the virulence of the pathogen responsible and host defenses. However, survival in these patients is determined primarily by the degree of severity at the time of pneumonia diagnosis and the response to initial therapy. As a consequence, the number of preventable deaths is likely to be much smaller than the total. Therapy requires both supportive and specific measures. When diagnostic information becomes available, it permits the rescue of some patients with inadequate therapy or simplifies the spectrum of the empirical therapy. Initial antibiotic choice should be based on expected etiologic pathogens, while knowledge of local microbial epidemiology and susceptibility patterns is crucial.