Coevolution between bacteria and bacteriophages can be characterized as an infinitive constant evolutionary battle (phage-host arm race), which starts during phage adsorption and penetration into host cell, continues during phage replication inside the cells, and remains preserved also during prophage lysogeny. Bacteriophage may exist inside the bacterial cells in four forms with different evolutionary strategies: as a replicating virus during the lytic cycle, in an unstable carrier state termed pseudolysogeny, as a prophage with complete genome during the lysogeny, or as a defective cryptic prophage. Some defensive mechanisms of bacteria and virus countermeasures are characterized, and some evolutionary questions concerning phage-host relationship are discussed.