About a century ago, bacteriophages or phages - viruses that infect bacteria - were discovered and reported in the scientific literature. This review aims at a comprehensive survey of bacteriophage discovery, research and applications since the 1920s, and its impact on molecular biology, biotechnology, health, ecology and the economy. Phage therapy has been proven a valuable asset to deal with pathogenic bacterial infections since the early 1920s, and has been practiced ever since, especially in the former Soviet Union and in Eastern Europe. The Western world remained sceptical and resorted to the widespread use of antibiotics since the 1940s. Now that antibiotic resistance among pathogenic bacteria has spread alarmingly and few really novel antibiotic compounds are in the pipeline, renewed attention is being directed to the use of phages as antibacterial agents in medicine. Because of this renewed interest in phage therapy in the Western world, novel applications with phages are being pursued in the human health, environmental and the agri-food sectors. This review will focus on (1) the history of early phage use and its successes and problems, (2) the study of phages as important tools in the development of molecular biology and biotechnology, (3) current developments in phage research including the use of phage endolysins for use in antibacterial treatment, (4) phage production systems including undesirable phage contamination of industrial fermentation processes based on bacteria, (5) recent applications in phage therapy and in phage-based control, and (6) the roles of phages in nature and in the human gut. (c) 2018 Society of Chemical Industry