This manual is one of the most comprehensive and authoritative works in the field of prokaryotic systematics. It is undergoing an extensive revision that will ultimately culminate in a five volume second edition. Arrangement of the content of the second edition follows the now familiar and well regarded phylogeny of the 16S rRNA gene, yet retains much of the layout of the first edition. Volume 1, encompassing the Archaea, Deeply Branching and Phototrophic Bacteria was published in 2001. Work on volume 2, The Proteobacteria, has been completed. This culminates a four year effort by Bergey's Manual Trust and more than 150 internationally recognized authorities to provide a comprehensive view of the Proteobacteria, the largest prokaryotic phylum.