Bacteriophage infection of starter cultures used in a range of milk fermentation processes, particularly those involving Lactococcus lactis, poses a significant problem in industrial practice. The application of genetic and molecular technologies to the study of lactococcal bacteriophages has proven to be very rewarding in terms of understanding the nature of phage with respect to their physical and genetic organisation. The availability of the full genomic sequence of a number of phages provides an unambiguous basis for determining the relationship between them, for elucidating their evolutionary progression and will also yield strategies for obstructing successful phage proliferation on previously sensitive hosts. The genetic analysis of phage/host interactions has also highlighted the presence of natural defence systems (e.g. adsorption blocking, inhibition of phage DNA entry, restriction modification and abortive infection) in lactococci. A number of restriction modification systems and abortive infection mechanisms have been characterized at a molecular level and the genes involved have been cloned and sequenced. Plasmid-encoded phage resistance mechanisms can be exploited to generate strains which can successfully counter phage proliferation and will provide a basis for understanding the complex interactions between phages and their target hosts at a molecular level.