Includes a description of the Alpha-, Beta-, Delta-, and Epsilonproteabacteria (1256 pages, 512 figures, and 371 tables). This large taxa include many well known medically and environmentally important groups. Especially notable are Acetobacter, Agrobacterium, Aquospirillum, Brucella, Burkholderia, Caulobacter, Desulfovibrio, Gluconobacter, Hyphomicrobium, Leptothrix, Myxococcus, Neisseria, Paracoccus, Propionibacter, Rhizobium, Rickettsia, Sphingomonas, Thiobacillus, Xanthobacter and 268 additional genera.