This review by David L. Kirchman examines the ecology of Cytophaga–Flavobacteria in aquatic environments, focusing on their role in carbon cycles and other biogeochemical processes. The Cytophaga–Flavobacteria cluster is one of the most abundant groups of heterotrophic bacteria in both freshwater and marine systems, known for their proficiency in degrading various biopolymers such as cellulose, chitin, and pectin. The review highlights the importance of molecular methods, such as fluorescence in situ hybridization (FISH) and PCR, in understanding the community structure and function of these bacteria. Despite their high abundance, the Cytophaga–Flavobacteria cluster is underrepresented in databases compared to other bacterial groups, which limits our understanding of their diversity and specific roles. The review also discusses the impact of grazing and viral lysis on the abundance of Cytophaga–Flavobacteria, suggesting that they may be preferentially grazed by bacterivores due to their high growth rates. Overall, the study emphasizes the need for more detailed research on the phylogenetic diversity and functional roles of Cytophaga–Flavobacteria to better understand their contributions to biogeochemical processes in aquatic ecosystems.This review by David L. Kirchman examines the ecology of Cytophaga–Flavobacteria in aquatic environments, focusing on their role in carbon cycles and other biogeochemical processes. The Cytophaga–Flavobacteria cluster is one of the most abundant groups of heterotrophic bacteria in both freshwater and marine systems, known for their proficiency in degrading various biopolymers such as cellulose, chitin, and pectin. The review highlights the importance of molecular methods, such as fluorescence in situ hybridization (FISH) and PCR, in understanding the community structure and function of these bacteria. Despite their high abundance, the Cytophaga–Flavobacteria cluster is underrepresented in databases compared to other bacterial groups, which limits our understanding of their diversity and specific roles. The review also discusses the impact of grazing and viral lysis on the abundance of Cytophaga–Flavobacteria, suggesting that they may be preferentially grazed by bacterivores due to their high growth rates. Overall, the study emphasizes the need for more detailed research on the phylogenetic diversity and functional roles of Cytophaga–Flavobacteria to better understand their contributions to biogeochemical processes in aquatic ecosystems.