Book excerpt: Bacteriologists from all levels of expertise and within all specialties rely on this Manual as one of the most comprehensive and authoritative works. Since publication of the first edition of the Systematics, the field has undergone revolutionary changes, leading to a phylogenetic classification of prokaryotes based on sequencing of the small ribosomal subunit. The list of validly named species has more than doubled since publication of the first edition, and descriptions of over 2000 new and realigned species are included in this new edition along with more in-depth ecological information about individual taxa and extensive introductory essays by leading authorities in the field.