Ruminants are characterized by the ability of microbes in the rumen to convert cellulosic feed into volatile fatty acids (VFAs) and proteins that meet the nutritional needs of the animal. Major advances in analytical techniques and genomics have transformed our understanding of rumen microbiology. This understanding is of critical importance to livestock production since rumen function affects nutritional efficiency, environmentally-damaging emissions from ruminants (such as methane and nitrous oxide) as well as animal health. Understanding and optimizing the function of rumen microbiota opens up the possibility of higher feed efficiency with fewer negative metabolic and environmental effects. This collection reviews what we know about rumen microbiota and the role of nutritional strategies in optimizing their function for more sustainable livestock production. The focus is on dairy cattle.