Few, if any, genes have had the same level of impact on the field of evolutionary-developmental biology (evo-devo) as the Hox genes. These genes are renowned for their roles in patterning the body plans and development of the animal kingdom. This is complemented by the distinctive organisation of these genes in the genome, with them frequently being found as clusters in which gene position is linked to when and where the individual genes are expressed, particularly during embryogenesis. This book provides the latest overviews of Hox gene organisation and function for major clades of animals from across the animal kingdom. With the rapidly increasing availability of high-quality whole genome sequences from an ever-expanding range of species, it is becoming increasingly evident that there is great diversity in the organisation of Hox genes. These great strides in genome sequencing are wedded to important developments in our ability to detect expression and disrupt gene function in species that are not traditionally genetically-amenable animals. These technical developments are integrated with wide taxon-sampling in this volume to provide new perspectives on the roles of Hox genes in understanding fundamental issues such as embryo patterning, mechanisms of gene regulation, homology, evolvability, evolutionary novelties, phylogeny, the role of gene and genome duplications in evolution, and ancestral states for major clades of animals.