Nosocomial infections are those which occur 48 hours after a person is being hospitalized. The Intensive Care Units (IUC) in newborns, pediatric, and adults are the most prevalent areas to get an intra hospital infection. This is due to the patient’s profile, therapies and invasive procedures that patients have to go through. Pneumonia is one of the most known and characterized hospital infections. It represents the second cause of nosocomial infection which increases morbid-mortality when it is associated to mechanical ventilation. The goal of this article is to show general aspects of pneumonia nosocomial; in particular those aspects associated with mechanical ventilation, the risks, pathogenesis, diagnostic criteria and different techniques used to take samples.