Through numerous reports of Pseudomonas aeruginosa outbreaks linked to transmission from environmental reservoirs, infection control practitionners have a univocal picture of its epidemiology: it is an opportunistic pathogen responsible of major outbreaks in intensive care units (ICU) with a major role played by the water network. The objective of this review was to answer to three questions: what is the part of hospital acquisition of P. aeruginosa? What is the part of outbreaks on incidence of hospital-acquired infections? What is the part of environment as a reservoir for transmission? Genotyping of hospital-acquired P. aeruginosa isolates allows us to define the endogenous or exogenous source of the infection and replace the concept of imported/acquired infection. If 80% of infections could be considered as acquired in ICU, the proportion of infections from exogenous source could be estimated at 50%. Even in a context of major outbreak, the epidemic clone represents 20% of the patients colonized and among these patients, only 50% are recognized with clinical samples. Some studies show that water fittings are a major source of P. aeruginosa in ICU. Other reports demonstrate a weak epidemiological link between environmental and clinical strains. Finally, despite the fact that the relative contributions of endogenous and exogenous sources to P. aeruginosa acquisition are not well established, we can assume that the epidemiological pattern of P. aeruginosa infection and colonization is not univocal and may vary both between ICU and within ICU depending on the period considered.