The gastrointestinal (GI) microbiota plays a crucial role in human health, and dietary approaches, such as consuming dietary fiber and prebiotics, are being explored to modulate its composition and metabolic function. These dietary components are not digested by human enzymes but are metabolized by gut microbes, producing short-chain fatty acids (SCFAs) like acetate, propionate, and butyrate. The article reviews the impact of fiber and prebiotic consumption on the GI microbiota, including the effects of fiber properties, adequate intake, and treatment dosages, as well as phenotypic responses related to the microbiota's composition. Dietary fiber, defined as carbohydrate polymers that are neither digested nor absorbed, can be classified into different types based on their origin, chemical composition, and physicochemical properties. Prebiotics, which selectively stimulate the growth and/or activity of specific bacteria, are a subset of dietary fibers. The fermentation of dietary polysaccharides by gut bacteria results in SCFAs, which have various physiological benefits, including influencing epithelial cell integrity, glucose homeostasis, lipid metabolism, appetite regulation, and immune function. The article highlights that the capacity of diet to modify the GI microbiota varies based on the type of fiber consumed. Different fibers have distinct fermentability, solubility, and viscosity, which influence microbial fermentation and therapeutic effects. For example, insoluble fibers like cellulose are poorly fermented but increase gut transit rate, while soluble fibers like β-glucan and pectins are highly fermentable and possess high solubility and viscosity. The impact of dietary fiber and prebiotics on the GI microbiota is influenced by factors such as fiber dosage, bacterial community composition, and individual genetics. Clinical studies have shown that specific fibers, such as galactooligosaccharides (GOS), inulin, and resistant starch, can induce blooms in beneficial bacteria like *Bifidobacterium* spp. and *Eubacterium*. However, the effects of different fibers on microbial metabolism can vary, and individual responses can differ due to factors like fiber dosage and the presence of keystone species in the microbiota. The article concludes by emphasizing the need for further research to understand the long-term health benefits of modulating the GI microbiota through dietary fiber and prebiotic consumption. Large prospective studies and well-controlled clinical trials are essential to establish causal relationships between perturbations in the microbiota and disease. Additionally, the integration of data from human, animal, and in vitro studies using advanced computational methods is recommended to enhance our understanding of host-microbe interactions.The gastrointestinal (GI) microbiota plays a crucial role in human health, and dietary approaches, such as consuming dietary fiber and prebiotics, are being explored to modulate its composition and metabolic function. These dietary components are not digested by human enzymes but are metabolized by gut microbes, producing short-chain fatty acids (SCFAs) like acetate, propionate, and butyrate. The article reviews the impact of fiber and prebiotic consumption on the GI microbiota, including the effects of fiber properties, adequate intake, and treatment dosages, as well as phenotypic responses related to the microbiota's composition. Dietary fiber, defined as carbohydrate polymers that are neither digested nor absorbed, can be classified into different types based on their origin, chemical composition, and physicochemical properties. Prebiotics, which selectively stimulate the growth and/or activity of specific bacteria, are a subset of dietary fibers. The fermentation of dietary polysaccharides by gut bacteria results in SCFAs, which have various physiological benefits, including influencing epithelial cell integrity, glucose homeostasis, lipid metabolism, appetite regulation, and immune function. The article highlights that the capacity of diet to modify the GI microbiota varies based on the type of fiber consumed. Different fibers have distinct fermentability, solubility, and viscosity, which influence microbial fermentation and therapeutic effects. For example, insoluble fibers like cellulose are poorly fermented but increase gut transit rate, while soluble fibers like β-glucan and pectins are highly fermentable and possess high solubility and viscosity. The impact of dietary fiber and prebiotics on the GI microbiota is influenced by factors such as fiber dosage, bacterial community composition, and individual genetics. Clinical studies have shown that specific fibers, such as galactooligosaccharides (GOS), inulin, and resistant starch, can induce blooms in beneficial bacteria like *Bifidobacterium* spp. and *Eubacterium*. However, the effects of different fibers on microbial metabolism can vary, and individual responses can differ due to factors like fiber dosage and the presence of keystone species in the microbiota. The article concludes by emphasizing the need for further research to understand the long-term health benefits of modulating the GI microbiota through dietary fiber and prebiotic consumption. Large prospective studies and well-controlled clinical trials are essential to establish causal relationships between perturbations in the microbiota and disease. Additionally, the integration of data from human, animal, and in vitro studies using advanced computational methods is recommended to enhance our understanding of host-microbe interactions.