Bacteriophages (viruses that infect bacteria) are fascinating organisms that have played and continue to play a key role in bacterial genetics and molecular biology. Phage can confer key phenotypes on their host for example, converting a non-pathogenic strain into a pathogen and they play a key role in regulating bacterial populations in all sorts of environments. The phage-bacterium relationship varies enormously, from the simple predator-prey model to a complex, almost symbiotic relationship that promotes the survival and evolutionary success of both. While infection of bacteria used in the fermentation industry can be very problematic and result in financial losses, in other scenarios, phage infection of bacteria can be exploited for industrial and/or medical applications. Interest in phage and phage gene products as potential therapeutic agents is increasing rapidly and is likely to have a profound impact on the pharmaceutical industry and biotechnology in general over the comi