Bacteria of the Acinetobacter genus received little attention for many years because of their weak pathogenic potential and changing taxonomy. Since the introduction starting in the 1980s of an ever increasing number of antimicrobials, these organisms have demonstrated their ability to adapt. They are causing an increasing number of nosocomial infections, most notably in intensive care units. The selection pressure exerted by antimicrobials and the use of increasingly invasive diagnostic and therapeutic procedures are the main factors that promote emergence of Acinetobacter in high-risk patients. Acinetobacter exhibit a high level of resistance to antimicrobials and are capable of persisting in hostile environments (humidity or dryness, presence of some antiseptics). As a result, they can cause nosocomial outbreaks. Identification of carriers and colonized patients, rigorous isolation, and scrupulous cleaning procedures are effective control measures. Despite these efforts, however, Acinetobacter baumannii now contributes a significant proportion of nosocomial infections.