Lactococcus lactis is extensively used in the production of cheese and cultured dairy products in industrial fermentations worldwide. Bacteriophage infection of L. lactis imposes a constant threat to the fermentation industry and represents the major cause of fermentation failure. Numerous phage defense strategies have been developed over the years to protect industrial starter cultures, particularly L. lactis. Numerous genes from lactococcal bacteriophages have been cloned and characterized and mechanisms that regulate their expression elucidated. Complete genome sequences of several L. lactis bacteriophages have also been determined. This accumulation of genetic information on lactococcal bacteriophages has led to a better understanding of the phage life cycle, host interactions, relationships between lactococcal phages, and possible patterns of phage evolution. These advances in molecular biology of lactococcal bacteriophages will be discussed with a view towards the development of novel and more effective phage defenses.