Effectively feeding forage and nonforage fiber to dairy cattle requires assessment of chemical and physical properties that determine ruminal fiber digestion and passage and dry matter intake. We will focus on the practical application of fiber digestibility in ration formulation and feeding management. Two historical transformative advances in fiber analysis and utilization were (1) the detergent system and summative approach to partition forages into fiber and nonfiber fractions, and (2) the concept that potentially digestible fiber follows first-order digestion kinetics. Dynamic nutrition models require kinetic measures of digestion and passage, and their required inputs will be an important driver of future fiber analyses. Research has focused on measuring multiple fiber digestion pools (i.e., fast, slow, and indigestible neutral detergent fiber; iNDF). Presumably, this 3-pool model better reflects biological reality of forage fiber digestion, but its practical significance in ration formulation has not been demonstrated. Of equal or greater importance is an understanding of multiple particle pools for passage, and forage factors such as particle size, density, and digestibility that influence passage rate. Estimation of iNDF by measurement of undigested fiber (uNDF) at long fermentation times is preferable to prediction from lignin because of the observed variation in the relationship between lignin and uNDF. Direct measures of uNDF in feed and feces will result in more accurate ration formulation and fiber utilization. Complementary analyses of nonfiber carbohydrates and fatty acids will be required to predict the impact of negative associative effects on NDF digestion. The nutritional value of cell wall monomer measurement is presently unknown. Laboratory analyses should assess the potential nutritive value of forage, but poor feeding management can reduce this potential. Future transformative analytical breakthroughs may connect plant anatomy and structure with laboratory measurements. Going forward, we need to better describe forage biology affecting ruminal digestion and passage with an analytical focus on fiber characteristics that allow formulation of rations with minimal, maximal, or optimal NDF content.