Viruses of microorganisms (VoMs) are the world's most abundant viruses. There are viruses for every known microbe and VoMs are usually described in terms of their hosts as algal viruses, archaeal viruses, bacteriophages, virophages, fungal viruses and protozoan viruses. A key feature of infection by VoMs is that they often kill the host. This allows VoMs to play a key role in modifying microbial communities and in nutrient cycling in various environments. When the host is itself a pathogen then VoMs may be exploited to create novel antimicrobial strategies. In fact phage therapy for a variety of antibiotic-resistant bacterial pathogens is currently at the clinical trial stage. When they don't kill the host, VoMs can still play important roles in the ecology and evolution of their hosts via various forms of virus-mediated horizontal gene transfer. Important in nature, these processes have also been used in the laboratory in genetic engineering techniques. In this multi-authored volume, international experts review the genomics, ecology, comparative biology and biotechnological applications of these fascinating viruses. Chapters have extensive reference sections that should encourage readers to pursue each subject in greater detail. This unique reference volume is a must-read for everyone working with VoMs, from the PhD student to the experienced scientist, in academia, the pharmaceutical or biotechnology industries and working in clinical environments.