Pseudomonas aeruginosa is responsible for an increasing proportion of infections acquired in the modern hospital setting. It accounts for 8.5% of all nosocomial infections and has an attack rate of 36 infections per 10,000 hospital discharges. P. aeruginosa represents the single most frequently isolated pathogen in patients with nosocomial pneumonia and burn-wound infections. The organism's bioepidemiology is linked to its ability to thrive in marginal econiches, and its ascendency as a nosocomial pathogen parallels the evolution of high-technology intensive care units, the large numbers of immunocompromised patients, and the liberal use of antibiotics. The reservoirs and modes of transmission for this organism are reviewed along with recent studies aimed at the prevention of both colonization and infection by this organism.