CRISPR-Cas is a recently discovered defense system which protects bacteria and archaea against invasion by mobile genetic elements such as viruses and plasmids. A wide spectrum of distinct CRISPR-Cas immune systems has been identified in at least half of the available prokaryotic genomes. On-going biochemical and functional analyses have resulted in substantial insight into the functions and possible applications of these fascinating systems, although many secrets remain to be uncovered. In this book, experts summarize the state of the art of this exciting field.