Publisher Summary Bacteriophages are viruses that multiply exclusively inside bacterial cells. In natural environments, bacteriophages form relatively stable associations with bacteria, called lysogenic that carry the virus intracellularly, but are resistant to its lytic action. Extracellular virus originates by the lysis of exceptional cells in lysogenic cultures. Genetic experiments with bacteriophage call for one or more stocks of phage and suitable bacterial hosts. A phage stock consists simply of the products of lysis of a sensitive bacterial culture infected with phage from a recently started clone. It may contain anywhere from 10 9 to 10 I2 phage particles per milliliter. The bacterial hosts required consists of a sensitive strain, used for the propagation and titration of the virus, and one or more indicator strains that are selectively resistant to certain viruses used for their identification. A series of dissimilar bacteriophages called T1 (Type 1), T2 to T7 all lysing a single strain of Escherichia coli known as B, have been used in all recent genetic experiments. Among these, only T2 and T4 have been intensively studied. These are related viruses, large in size, which possess morphologically and functionally differentiated structure.