Hospital-acquired pneumonia (HAP) remains a significant cause of morbidity and attributable mortality, especially among patients undergoing mechanical ventilation. The clinical approach to this disorder continues to evolve. Although our understanding of the epidemiology, risk factors, and pathogenesis of this disorder are expanding, consensus on diagnostic, therapeutic, and preventive strategies is lacking. Although outcome is significantly improved by the rapid introduction of appropriate antimicrobial therapy, presently available diagnostic tests rarely are able to identify a specific pathogen when antimicrobial choices are made. Thus, most therapy is by necessity empirical. The American Thoracic Society (ATS) published guidelines for the empiric treatment of HAP in 1996, this article reviews the recommendations of these guidelines and, if new information is available, updates these recommendations.