Pseudomonas aeruginosa chronic lung infections are the major cause of morbidity and mortality associated with cystic fibrosis. For many years, the consensus was that cystic fibrosis patients acquire P. aeruginosa from the environment, and hence harbour their own individual clones. However, in the past 15 yrs the emergence of transmissible strains, in some cases associated with greater morbidity and increased antimicrobial resistance, has changed the way that many clinics treat their patients. Here we provide a summary of reported transmissible strains in the UK, other parts of Europe, Australia and North America. In particular, we discuss the prevalence, epidemiology, unusual genotypic and phenotypic features, and virulence of the most intensively studied transmissible strain, the Liverpool epidemic strain. We also discuss the clinical impact of transmissible strains, in particular the diagnostic and infection control approaches adopted to counter their spread. Genomic analysis carried out so far has provided little evidence that transmissibility is due to shared genetic characteristics between different strains. Previous experiences with transmissible strains should help us to learn lessons for the future. In particular, there is a clear need for strain surveillance if emerging problem strains are to be detected before they are widely transmitted.