Insects are the most diverse, and arguably the most successful, group of animals on Earth and they make significant contributions to, amongst other things, human disease and agriculture. It is now clear that a large part of the success of insects is due to the symbiotic micro-organisms that they carry both intra- and extracellularly (e.g as endosymbionts or in the gut, respectively). In this book the Editors have assembled a group of contributors to provide chapters on a range of topics from a description of the endosymbionts of lice to the genomics and transcriptomics of mutualism and the phylogeny of insect symbionts. These chapters are like reviews and provide a good, and accurate, background of the topic under discussion. The wide-ranging nature of the content ensures that this book has something for everyone with an interest in symbiosis (aka bacteria-host interactions) and it would be a useful addition to the bookshelf of any researcher with an interest in symbiosis in general.