A combination of animal genetics and the unique, enlarged fore-stomach of ruminants (rumen) enable domesticated ruminants to be sustained on forages and fibrous feedstuffs that would be otherwise indigestible. Ruminants can also utilise more easily digestible, high energy plant material such as grain, to achieve rapid increases in weight gain, muscle bulk and in the case of dairy cows, high milk yields. Since the mid-1900s there has been a steady research effort into understanding the digestive processes of ruminants, striving to maintain animal health and nutrition whilst maximising the productivity and environmental sustainability of livestock production systems. This article describes strategies developed to modulate the rumen microbial ecosystem, enabling the utilisation of plant feedstuffs that may otherwise be toxic and enhancing feed utilisation efficiency or controlling populations of specific rumen microbes, such as those contributing to lactic acidosis and enteric methane emissions. It also traces advances in technologies that have enabled us to understand the underlying biological mechanisms involved in the modulation of the rumen microbiome.