Bacteriophages or phage are, like other viruses, obligate intracellular parasites which infect and replicate within bacterial cells. Bacteria and phages are changing – bacteria can evolve resistance to phage attack and defend themselves against phage infections and phage can increase infectivity against bacteria. This process is called coevolution and causes genetic and phenotypic changes. We know two types of coevolution: arms race dynamic and fluctuating selective dynamic. Bacteria have evolved a number of defense mechanisms against phages, including prevention of phage adsorbtion, prevention of phage DNA entry, cutting phage nucleic acid or initiation of cell death upon infection. Phages have been known to evolve mechanisms to evade bacteria defence and increase infectivity. Continuous processes have the best conditions for coevolution because bacteria and phage are in interaction long enough to start the process of coevolution. Plasmids, mutations rates, shaking growth culture, bacterial and phage migrations and abiotic factors like fluctuating environment affect the bacteria-phage coevolution. Bacteria-phage coevolution process is also exploited at bacteriophage therapy.