Bacteriophages are viruses that infect and, in many cases, destroy their bacterial targets. Within a few years of their initial discovery they were being investigated as therapeutic agents for infectious disease, an approach known as phage therapy. However, the nature of these exquisitely specific agents was not understood and much early use was both uninformed and unsuccessful. As a result they were replaced by chemical antibiotics once these became available. Although work on phage therapy continued (and continues) in Eastern Europe, this was not conducted to a standard allowing it to support clinical uses in areas regulated by the European Medicines Agency or the US FDA. To develop phage therapy for these areas requires work carried out in accordance with the requirements of these agencies, and, driven by the current crisis of antibiotic resistance, such clinical trials are now under way. The first Phase I clinical trial of safety was reported in 2005, and the results of the first Phase II clinical trial of efficacy of a bacteriophage therapeutic was published in 2009. While the delivery of these relatively large and complex agents to the site of disease can be more challenging than for conventional, small-molecule antibiotics, bacteriophages are then able to multiply locally even from an extremely low (picogram range) initial dose. This multiplication where and only where they are needed underlies the potential for bacteriophage therapeutics to become a much needed and powerful weapon against bacterial disease.