The fiber component of the diet is the major food source for the intestinal microbiota of swine. Included are traditional insoluble (e.g., cellulose) and soluble (e.g., pectin) dietary fibers, resistant starches (four types), and oligosaccharides (some of which are “prebiotics”). Fiber constituents are found both in the primary and secondary cell walls of plants, but some are found in non-cell wall structures as well. In cereals, cell walls consist of a reinforced multi-component matrix of cross-linked polymers (acidic xylans, arabinoxylans, glucomannans) in which a network of cellulose microfibrils is embedded. Proteins form a second network in the matrix, and lignin and phenolic acids may be covalently linked to the matrix polysaccharides. In addition, whole grains have considerable amounts of resistant starch. Physical processing and cooking have a major effect on the chemical composition and subsequent utilization of the whole grain. Techniques are available to quantify both the insoluble and soluble fibers present in swine diets. Solubility often impacts the location of fermentation within the swine gastrointestinal tract, and solubility often, but not always, reflects fermentation potential. Other important attributes of fiber include its viscosity/gel-forming capacity/water-holding capacity, and its fermentability. On the analytical front, considerable progress has been and continues to be made, with many AOAC-approved techniques currently available.