The ultimate measure of forage quality is animal performance, which depends upon nutrient concentration in the forage, intake by the animal and digestibility and metabolism of the nutrients consumed. Because forage quality can be determined only by animals it is pertinent to understand how animal, forage and feeding situation affect in vivo measurements of forage quality. Most variation in animal performance is related to intake, which is influenced by both the animal and forage. When forage is high in digestible energy, the energy demand of the animal determines intake. When forage is high in fiber, intake is limited by the bulkiness of the forage in filling the stomach and the time required for mastication and passage. Caution must be exercised when estimating intake based only on forage measurements because the animal has significant impact on intake. Intake is affected by animal species, physiological status, level of refusal, and physical form of the forage. Because ruminants are designed to maximize forage digestion, digestibility varies little among animals, and variation in digestibility is related primarily to forage characteristics. Non-fiber fractions of forages are highly digestible; therefore, dietary fiber and fiber digestibility determine the total dry matter digestibility of forages. Although lignin affects fiber digestibility, it is estimated more accurately by in vitro “artificial rumen” procedures. Methodology affects in vitro results, and variation among laboratories often limits practical application. Estimates of intake and digestibility are often combined into a single index of forage quality, such as relative feed value and relative forage quality. Future forage evaluation systems will be based on chemical, physical and biological descriptions of forages that can be used with digestion kinetics to define the unique forage quality associated with a specific forage that is fed in a particular situation to a given type of animal.