The golden age of antibiotics, which lasted from the 1930s until 2005, brought a brisk clip of antibiotic discovery and fueled optimism about the victory of modern medicine over bacterial infections. Since then, however, with a stalled antibiotic discovery effort and wide-spread antibiotic use, antimicrobial resistance has emerged as a major global health threat. Bacteriophages, or phages (literally viruses that infect certain bacteria), have coevolved with bacteria for almost 4 billion years and are the most abundant organisms on the earth. Substantial progress is being made such that selection, engineering, and synthetic production of phages may make it possible for these lethal enemies of bacteria to be harnessed as potent allies in our battle against antimicrobial resistance.