In 1917, bacteriophages were recognized as epizootic infections of bacteria and were almost immediately deployed for antibacterial therapy and prophylaxis. The early trials of bacteriophage therapy for infectious diseases were confounded, however, because the biological nature of bacteriophage was poorly understood. The early literature reviewed here indicates that there are good reasons to believe that phage therapy can be effective in some circumstances. The advent of antibiotics, together with the "Soviet taint" acquired by phage therapy in the postwar period, resulted in the absence of rigorous evaluations of phage therapy until very recently. Recent laboratory and animal studies, exploiting current understandings of phage biology, suggest that phages may be useful as antibacterial agents in certain conditions.