The article reviews the role and mechanisms of *Bacteroidetes* in polysaccharide degradation across various habitats, including the gastrointestinal tract (GIT) of animals and environmental niches such as soil, ocean, and freshwater. *Bacteroidetes* are diverse bacteria that play crucial roles in degrading complex organic matter, particularly polysaccharides, which are abundant in diets of humans and animals. They are well-adapted to degrade plant cell wall compounds and host-derived carbohydrates, contributing to the energy and nutrient supply of their hosts. The phylum is also found in the normal microbiota of animals, where it interacts with the host in a mutualistic manner, aiding in the development of the GIT and immune system. However, some *Bacteroidetes* can act as pathogens, causing infections in various hosts, including humans, animals, and plants. The article highlights the versatility of *Bacteroidetes* in degrading complex biopolymers and their role in carbon fluxes in ecosystems. Additionally, it discusses the genomic perspective of *Bacteroidetes*, noting the presence of numerous polymer-degrading enzymes and the evolution of polysaccharide utilization loci (PULs). The diet of animals is found to significantly influence the composition of the gut microbiota, with dietary changes affecting the abundance and diversity of *Bacteroidetes* species. Overall, the article emphasizes the importance of *Bacteroidetes* in both ecological and host interactions, highlighting their adaptability and specialization in degrading complex organic matter.The article reviews the role and mechanisms of *Bacteroidetes* in polysaccharide degradation across various habitats, including the gastrointestinal tract (GIT) of animals and environmental niches such as soil, ocean, and freshwater. *Bacteroidetes* are diverse bacteria that play crucial roles in degrading complex organic matter, particularly polysaccharides, which are abundant in diets of humans and animals. They are well-adapted to degrade plant cell wall compounds and host-derived carbohydrates, contributing to the energy and nutrient supply of their hosts. The phylum is also found in the normal microbiota of animals, where it interacts with the host in a mutualistic manner, aiding in the development of the GIT and immune system. However, some *Bacteroidetes* can act as pathogens, causing infections in various hosts, including humans, animals, and plants. The article highlights the versatility of *Bacteroidetes* in degrading complex biopolymers and their role in carbon fluxes in ecosystems. Additionally, it discusses the genomic perspective of *Bacteroidetes*, noting the presence of numerous polymer-degrading enzymes and the evolution of polysaccharide utilization loci (PULs). The diet of animals is found to significantly influence the composition of the gut microbiota, with dietary changes affecting the abundance and diversity of *Bacteroidetes* species. Overall, the article emphasizes the importance of *Bacteroidetes* in both ecological and host interactions, highlighting their adaptability and specialization in degrading complex organic matter.