Ventilator-associated pneumonia (VAP) is a pulmonary infection that occurs after at least 48 hours of mechanical ventilation (MV). The incidence depends on several factors, although the most important are those related to the host and duration of MV. VAP can be differentiated into early-onset (<5 days) and late-onset types (> or =5 days). The overall incidence of VAP varies between 9% and 70% (average, 20% to 25%), and the majority of episodes occur within the first 5 days. Risk factors for VAP include prolonged MV, older age, supine body position, and type of comorbidity. Oropharyngeal colonization appears to be a risk factor for early-onset pneumonia, whereas prolonged MV and antibiotic pretreatment, especially with broad-spectrum drugs, increase the risk for late-onset VAP Microaspiration of colonized oropharyngeal secretions is a major cause of early-onset VAP, most frequently caused by community-type pathogens. After 5 days of MV, pathological colonization with gram-negative bacteria may occur, and late-onset VAP is more likely to be attributable to this group of microorganism. Incidence, risk factors, and microbiology depend strongly on the time frame in which the episode develops. However, initial and pathological colonization during the intensive care unit stay can modify this concept.