This section of the article provides an overview of the second edition of Bergey's Manual® of Systematic Bacteriology, Volume Two, focusing on the Proteobacteria. It highlights the contributions of various editors and contributors, emphasizing the dynamic nature of systematic prokaryotic biology and the reliance on phylogenetic frameworks based on nucleotide sequence analysis. The manual is structured into several orders and families, detailing the classification and characteristics of various bacterial genera within the Alpha-, Beta-, Delta-, and Epsilonproteobacteria. The content includes detailed descriptions of bacterial species, their taxonomic relationships, and ecological roles, reflecting the latest advancements in sequencing technology and taxonomic research. The section also acknowledges the support of numerous institutions and individuals who have contributed to the preparation and publication of the manual.This section of the article provides an overview of the second edition of Bergey's Manual® of Systematic Bacteriology, Volume Two, focusing on the Proteobacteria. It highlights the contributions of various editors and contributors, emphasizing the dynamic nature of systematic prokaryotic biology and the reliance on phylogenetic frameworks based on nucleotide sequence analysis. The manual is structured into several orders and families, detailing the classification and characteristics of various bacterial genera within the Alpha-, Beta-, Delta-, and Epsilonproteobacteria. The content includes detailed descriptions of bacterial species, their taxonomic relationships, and ecological roles, reflecting the latest advancements in sequencing technology and taxonomic research. The section also acknowledges the support of numerous institutions and individuals who have contributed to the preparation and publication of the manual.