This review discusses the ecology of the Cytophaga–Flavobacteria cluster in aquatic environments. It highlights the diversity and abundance of this bacterial group, which is a major component of prokaryotic biomass in surface waters. The Cytophaga–Flavobacteria cluster is a diverse group of bacteria that includes several genera, such as Cytophaga, Flavobacterium, and Bacterioides. These bacteria are often found in marine and freshwater environments and are known for their ability to degrade complex organic materials like cellulose, chitin, and pectin. The review also addresses the challenges in studying these bacteria due to their uncultivated nature. Molecular methods, such as fluorescence in situ hybridization (FISH) and PCR, are used to examine their distribution and abundance in various aquatic habitats. However, there are limitations in the current understanding of their taxonomy and ecological roles. For example, FISH probes may not always recognize all members of the Cytophaga–Flavobacteria cluster, leading to underestimation of their abundance. Additionally, clone libraries may not accurately represent the diversity of these bacteria, as they may be biased towards certain groups. The review also discusses the role of Cytophaga–Flavobacteria in the degradation of organic material in aquatic environments. These bacteria are particularly effective at breaking down high molecular weight dissolved organic matter (DOM), which is a major source of organic carbon in the biosphere. They are often found in high abundance in marine environments and play a significant role in the microbial loop, contributing to the mineralization of organic matter. The review also addresses the growth rates of Cytophaga–Flavobacteria and other bacterial groups, noting that their high growth rates may contribute to their abundance in certain environments. However, the impact of grazing by protists and viruses on these bacteria is not fully understood. The review concludes that further research is needed to better understand the ecological roles of Cytophaga–Flavobacteria and other uncultivated bacterial groups in aquatic environments. This includes improving our understanding of their taxonomy, ecology, and biogeochemical functions.This review discusses the ecology of the Cytophaga–Flavobacteria cluster in aquatic environments. It highlights the diversity and abundance of this bacterial group, which is a major component of prokaryotic biomass in surface waters. The Cytophaga–Flavobacteria cluster is a diverse group of bacteria that includes several genera, such as Cytophaga, Flavobacterium, and Bacterioides. These bacteria are often found in marine and freshwater environments and are known for their ability to degrade complex organic materials like cellulose, chitin, and pectin. The review also addresses the challenges in studying these bacteria due to their uncultivated nature. Molecular methods, such as fluorescence in situ hybridization (FISH) and PCR, are used to examine their distribution and abundance in various aquatic habitats. However, there are limitations in the current understanding of their taxonomy and ecological roles. For example, FISH probes may not always recognize all members of the Cytophaga–Flavobacteria cluster, leading to underestimation of their abundance. Additionally, clone libraries may not accurately represent the diversity of these bacteria, as they may be biased towards certain groups. The review also discusses the role of Cytophaga–Flavobacteria in the degradation of organic material in aquatic environments. These bacteria are particularly effective at breaking down high molecular weight dissolved organic matter (DOM), which is a major source of organic carbon in the biosphere. They are often found in high abundance in marine environments and play a significant role in the microbial loop, contributing to the mineralization of organic matter. The review also addresses the growth rates of Cytophaga–Flavobacteria and other bacterial groups, noting that their high growth rates may contribute to their abundance in certain environments. However, the impact of grazing by protists and viruses on these bacteria is not fully understood. The review concludes that further research is needed to better understand the ecological roles of Cytophaga–Flavobacteria and other uncultivated bacterial groups in aquatic environments. This includes improving our understanding of their taxonomy, ecology, and biogeochemical functions.