Understanding of ruminant digestive function has improved tremendously over the past 25 yr. This progress, viewed in retrospect, has clearly led to improvements in animal production. The encouraging aspect of this is that these improvements were made through qualitative application of our knowledge. Quantitative application of this knowledge-a major emphasis in current efforts--will likely yield even greater benefits in the future. Although we continue to identify new rumen microbes, it is suggested that the major rumen microbes have been identified. The nutritional requirements, physiological characteristics, biochemical pathways and functions of these microbes have been largely characterized. Major strides have been made toward understanding the complex interactions among rumen microbes that characterize the rumen ecosystem; microbes interact to improve the digestion of complex carbohydrates, such as cellulose, beyond the extent individual species can achieve; microbes provide nutrients required for the growth of other microbes and microbes remove products inhibitory to other microbes usually to their mutual benefit. Considerable progress has been made toward development of quantitative relationships among the chemical composition of ruminant feeds, dynamic aspects of digestion in the rumen, products of digestion absorbed by the ruminant, and, most important, how these can be manipulated to improve animal productivity.