Bacteriophages (viruses that infect bacteria) span the planet in extraordinary numbers. From oceans to deserts to the human body, metagenomics is uncovering the incredible genetic diversity, distributions, and ecology of bacteriophage populations. We continue to learn about how these viruses impact bacterial mortality, gene transfer, and nutrient cycling. Bacteriophage infections can even alter the phenotype of their hosts. Understanding bacteriophage diversity, ecology, and coevolution with their hosts will be critical to identifying new molecular biology tools, investigating the feasibility of phage therapy, and more accurately modeling global biogeochemical cycling.