Pneumonia is the second most common nosocomial infection in the Unites States with the highest rates seen in patients requiring mechanical ventilation. It is the leading cause of death among patients with hospital acquired infection with an over all mortality of at least 30%. Most nosocomial pneumonias are caused by Gram negative bacilli, which represent 50 – 70% of infection, Pseudomonas aeruginosa being the most common, while in Gram positive cocci, Staphylococcus aureus especially methicillin resistant Staphylococcus aureus is the most common organism as it accounts for 15 – 30% of nosocomial pneumonia. Viruses and fungi are less important causes of nosocomial pneumonia. Accurate diagnosis of nosocomial pneumonia along with identification of responsible organisms can be challenging. This task becomes even more difficult in patients who are mechanically ventilated. Prevention of nosocomial pneumonia is the most important step towards reducing hospital costs.