The host range of a bacteriophage is defined by what bacterial genera, species and strains it can lyse; it is one of the defining biological characteristics of a particular bacterial virus. Because of host factors such as masking by O antigens that affects injection and the presence of restriction endonucleases, the relative efficiency of plating (EOP), that is, the titer of the phage on a given bacterial cell line compared to the maximum titer observed, may vary considerably. This chapter describes rapid procedures for determining the host range and relative EOP on each host of any phage.