The appropriate selection of definitive antimicrobial therapy is a necessary component of the overall treatment for ventilator-associated pneumonia. When possible, single-agent therapy is preferable. A combination of antibiotics is necessary to treat multiple organisms not susceptible to a single appropriate antibiotic and when antibiotic-resistant gram-negative bacteria are present. Treatment failure is more commonly the result of persistent pneumonia and the development of antibiotic resistance than to recurrence after successful antimicrobial therapy. The duration of treatment will vary depending on the severity of the underlying illness and the pneumonic process.