Pneumonia is a constellation of symptoms and signs in combination with at least one opacity on chest radiography. Pneumonia affects 4 million adults per year in the USA (with the highest rates at the extremes of age and during the winter months), about 20% of whom are admitted to a hospital for treatment; in fact pneumonia is the sixth leading cause of death and the most common lethal infectious disease with an annual cost of 10 billion. Pneumonia can be broadly categorized as: community acquired pneumonia, health -care associated, hospital acquired (nosocomial) pneumonia and ventilator-associated. These categories provide a rough guide as to likely pathogens, disease severity and treatment. In light of the significant morbidity and potential mortality of pneumonia, appropriate measures of prevention should be instituted as: smoking cessation, optimising the patient's nutritional status and current pneumococcal and flu vaccines. Because the microbiological etiology of pneumonia is frequently unknown, initial antibiotic therapy is often empirical. There are currently three sets of North American guidelines for empirical antibiotic treatment of pneumonia. Under current guidelines, patients are stratified with respect to where treatment is initiated, the presence of underlying cardiopulmonary disease and other modifying factors such as whether the patient is likely to be infected with drug resistant S. pneumoniae, gram negative enteric bacilli or P. aeruginosa.