Publisher Summary Intensive study of bacteriophages during the past 15 years has shown fairly conclusively that the phages are the most promising objects of study for virologists seeking basic information about the life cycle of viruses. The rather well-known evidence for the preceding statement rests upon the fact that: (1) the host, the bacterium, is an isolatable single-celled organism; (2) the accuracy of assay of both the phages and the host cells; and (3) the speed, with which experiments can be carried out. The coliphage system is surprisingly broad. It includes four different serological groups that are paralleled by four distinct morphologies: at least two different types of mutual interaction between the phages adsorbed to the same bacterium and judging from the evidence available at the moment of writing, possibly two distinct methods of multiplication. At the outset, it should be stressed that it is operationally and experimentally deceptive to talk about phages and to be thinking of the physicochemical particles revealed by the electron microscope. An extracellular phage particle is a nonmetabolizing, nonmultiplying entity; only when it has contacted and merged with a susceptible cell, it has the ability to multiply. The evidence to be discussed in this chapter points strongly to the conclusion that the adsorbed phage particle undergoes such a profound alteration shortly after adsorption as to render almost meaningless the identification of it with the extracellular particle. The experimental findings (and the analogy) point up the necessity of knowing the properties of the extracellular particle, for the nature of the end-product is obviously an important clue to and a necessary limitation on the nature of the machinery of multiplication. Accordingly, after a brief discussion of basic biological properties of the phages, the physical, chemical, and genetic aspects of phages are presented in this chapter. Finally, the life cycle of the phages are outlined in some detail.