This review describes the progress which has been made during the last 10 to 15 years in the field of rumen microbiology. It is basically an account of new discoveries in the bacteriology, protozoology, biochemistry, and ecology of the rumen microbial population. As such it covers a wide range of subjects including the isolation and properties of methanogenic bacteria, the role of rumen phycomycete fungi, anaerobic energy conservation, and general metabolic aspects of rumen microorganisms. It also attempts, however, to describe and develop new concepts in rumen microbiology. These consist principally of interactions of the microbe-microbe, microbe-food and microbe-host types, and represent the main areas of recent advance in our understanding of the rumen ecosystem. The development of experimental techniques such as chemostat culture and scanning electron microscopy are shown to have been instrumental in progress in these areas. The paper is concluded with an assessment of our present knowledge of the rumen fermentation, based on the degree of success of experiments with gnotobiotic ruminants inoculated with defined flora and in mathematical modelling of the fermentation. The efficacy of chemical manipulation of the fermentation in ruminant is also discussed in this light.