Bacteriophage infection of lactic acid bacteria starter cultures can result in serious disruption or even failure of fermentation processes. With the help of newly developed techniques, progress has been made in the molecular characterisation of phages of lactic acid bacteria (LAB) and also in the elucidation of the nature of their interaction with host cultures. A number of receptors involved in phage adsorption have been indentified and DNA penetration and injection, as well as the intracellular development of the LAB phages have been investigated. The structural organisation of Lactobacillus and lactococcal phage genomes have also been determined and a number of phage genes has been cloned and sequenced. These include determinants for phage lysin and phage structural proteins. Specific loci, attachment sites (att), involved in the integration of temperate phage genomes, the cohesive ends (cos), involved in phage genome circularisation and the packaging sites (pac) of circularly permuted phages have been localised and in many cases characterised at a molecular level.