The purpose of this book is to argue, and to convince those who may not agree, that the Hox genes are indeed so important that their study will not end with the 20th century. The book is divided into three major sections. The first section covers aspects of the regulation of Hox gene expression and the structure and function of the now justifiably well-known homeobox. The second section offers insights and discussions of the sometimes contentious issues of the origin and evolution of the aforementioned Hox complexes. In the third and last section the role of the resident loci in the specification of body plans and meristic identity of thearthropods is presented.