Since the initial 1996 American Thoracic Society (ATS) guideline on nosocomial pneumonia, a number of new developments have appeared, mandating a new evidence-based guideline for hospital-acquired pneumonia (HAP), including healthcare-associated pneumonia (HCAP) and ventilator-associated pneumonia (VAP). This document, prepared by a joint committee of the ATS and Infectious Diseases Society of America (IDSA), focuses on the epidemiology and pathogenesis of bacterial pneumonia in adults, and emphasizes modifiable risk factors for infection. In addition, the microbiology of HAP is reviewed, with an emphasis on multidrug-resistant (MDR) bacterial pathogens, such as Pseudomonas aeruginosa, Acinetobacter species, and methicillin-resistant Staphylococcus aureus. Controversies about diagnosis are discussed, emphasizing initial examination of lower respiratory tract samples for bacteria, and the rationale for both clinical and bacteriologic approaches, using either" semiquantitative" or" quantitative" microbiologie methods that help direct selection of appropriate antibiotic therapy. We also provide recommendations for additional diagnostic and therapeutic evaluations in patients with nonresolving pneumonia. This is an evidence-based document that emphasizes the issues of VAP, because there are far fewer data available about HAP in nonintubated patients and about HCAP. By extrapolation, patients who are not intubated and mechanically ventilated should be managed like patients with VAP, using the same approach to identify risk factors for infection with specific pathogens.