Excessive intake of fat was a risk factor for chronic diseases like obesity, diabetes, and metabolic syndrome. However, removing fat from foods was challenging because of its critical role in providing desirable appearances, textures, mouthfeels, and flavors. This challenge could be partly overcome by using polysaccharide-based colloids to mimic some of the physicochemical attributes normally provided by fat, thereby improving the sensory attributes of reduced-fat foods. Various kinds of covalent and non-covalent interactions between polysaccharides and other food components (especially proteins and polyphenols) are reviewed. The effects of these interactions on the properties of polysaccharide-based colloids and on the quality of reduced-fat foods are then discussed. In addition, the fat replacement mechanism of polysaccharide-based colloids is briefly summarized. The interaction of polysaccharides with other food components can be used to create a range of colloidal fat replacers that can simulate some of the desirable characteristics normally provided by fats in foods (e.g., light scattering, texture modification, lubrication, and flavor retention/release). However, the assembly of these colloidal fat mimetics must be carefully controlled to obtain the required functional attributes. In addition, these interactions can contribute to the quality of fat-reduced foods by improving the emulsification, water holding and gelling properties of polysaccharides. In summary, polysaccharide-based ingredients offer an effective means of enhancing the appearances, feel, flavor, and health benefits of reduced-fat foods.