Bacteriophage (phages) are viruses that exclusively use bacterial cells for propagation, killing the bacterial host in the process. In phage therapy, phages are used to reduce bacterial numbers, thereby curing bacterial infections. Although this principle is conceptually straightforward, its practical application faces several hurdles. In this Primer, the practical aspects of phage therapy are outlined. We introduce the microbiological methods used to prepare and characterize phages and elucidate their interactions with bacteria. The discussion covers how the information in complete phage genome sequences is used, along with how RNA sequencing can enhance our understanding of phage biology. Selection parameters for therapeutic phages for clinical applications and key elements in industrial-scale phage production are provided. A summary of clinical trials both past and present, phage administration and dosing issues is analysed, as well as limitations associated with phage therapy and mitigation strategies. Finally, we speculate on the future of phage therapy.