Bacteriophages are viruses whose hosts are bacterial cells. Like all viruses, phages are metabolically inert in their extra-cellular form, reproducing only after infecting suitable host bacteria. Discovered over 80 years ago, they have played a key role in the development of modern biotechnology. Their initial isolation appeared to offer the first therapeutic for the control of infectious disease. The discovery of antibiotics in the 1940s eclipsed bacteriophage-based therapies although, with the increase in multiply drug-resistant pathogens, bacteriophages are being re-evaluated as the basis of new therapeutic strategies. Their defined host specificity facilitated their application in the typing and identification of a wide range of bacteria. Bacteriophage typing schemes were developed for most groups of pathogenic of bacteria and more recently their host specificity has been applied to the development of bacterial detection and diagnostic strategies. The advance in molecular biology over the past 30 years has been built on the study of phage structure and genetics carried out through the 1950s and 1960s. Restriction endonucleases which form the basis of molecular cloning were developed following studies of phage infection and many phage enzymes provide tools for the molecular biologist. The technique ofphage display has more recently provided a powerful technique for the identification and optimisation of ligands for antibodies and other biomolecules. In the environment they have been widely applied as tracers, as indicators of pollution and in the monitoring and validation of biological filters. While providing a valuable resource to the development of modern biotechnology, their ability to mobilise and transfer toxin genes in the environment is viewed with concern. They also present a continuing challenge to the fermentation and in particular, the dairy industry, where phage infection can prove commercially disastrous.