Voluntary dry matter intake (VDMI) of forages by ruminants may be limited by distention resulting from restricted flow of digesta through the gastrointestinal tract. An animal's capacity for fill depends on the weight and volume of digesta that causes distention and the flow rate of digesta from the organ in which distention occurs. The reticulorumen is generally regarded as the site in the gastrointestinal tract for which distention limits VDMI with high-fill diets, although evidence suggests that distention of the abomasum may also limit VDMI. Linear decreases in VDMI have been noted with increasing amounts of inert fill inserted into the reticulorumen, but results have not been consistent across several experiments. Reduction in VDMI depends on the extent to which intake is limited by fill before insertion of inert fill; hence animals with high energy requirements consuming relatively low-energy, high-fill diets are affected to the greatest extent. Because NDF generally ferments and passes from the reticulorumen more slowly than other dietary constituents, it has a greater filling effect over time than non-fibrous feed components and has been found to be the best single chemical predictor of VDMI. However, many other factors affect fill, including particle size, chewing frequency and effectiveness, particle fragility, indigestible NDF fraction, rate of fermentation of the potentially digestible NDF, and characteristics of reticular contractions. These factors are only partially accounted for in models that have been developed to predict VDMI. Increased accuracy of prediction of VDMI is expected as models continue to evolve.