Nosocomial infection is one of the major health problems confronting clinicians in intensive care units. Incidence has been estimated to vary from 5 to 10%, leading to death in one of every 5000 persons hospitalized. This situation is not however static since hospital epidemics and nosocomial infections evolve as a consequence of advances in antibiotic drug therapy and surgical and intensive care techniques. Microbial investigations themselves are of major importance as the collection of epidemiological data and its communication to hospital partners affects their approach to patient management. Nosocomial pneumonia, particularly in ventilated patients, urinary infection in those with indwelling catheters and wound colonization after surgery are still major problems and explain part of the variations between wards and hospitals. The main changes concern the bacteria involved. Classical agents with new mechanisms of resistance (Klebsiella pneumoniae, extended spectrum beta-lactamase producers) have been joined by other "new" agents emerging as pathogens (Xanthomonas maltophilia, Acinetobacter baumannii). The incidence of resistance in Staphylococcus and Enterococcus strains as well as new Gram positive species including Corynebacterium jeikeium and Rhodococcus equi, now recognized as opportunistic agents, has also increased. The multiresistance of these pathogens is often responsible for failure of antibiotic therapy and emphasizes the need both for rigorous prevention through active cooperation between bacteriologists, ward clinicians and intensive care units, and further research in the fields of microbial resistance mechanisms and antibiotics. Preventive measures have become a priority creating an international challenge to health care policy makers.