Proteobacteria are a major group (phylum) of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera. Others are free-living, and include many of the bacteria responsible for nitrogen fixation. This book presents current research from across the globe in the study of proteobacteria, including phylogeny and activity of proteobacteria in sediments from Lake Furnas; proteobacteria forming nitrogen-fixing symbiosis with higher plants; proteobacteria and the endosymbiotic origin of mitochondrion; and interpreting diversity of proteobacteria based on 16S rRNA gene copy number.