Ruminant livestock are responsible for production of a portion of greenhouse gases, particularly methane (61Tg/yr) which is believed to contribute to global warming and climate change. Methane is an end product of fermentation of plant material by the microbial ecosystem in the rumen. Methanogenesis is undertaken by methanogenic archaea and is a mechanism by which H2 is removed from fermentation in order to regenerate biochemical co-factors such as NAD+. The microbial ecosystem is very complex and involves thousands of species of bacteria (1010-1011cells/ml), archaea (107-109cells/ml), protozoa (104-106cells/ml), fungi (103-106cells/ml), and viruses (109-1010cells/ml), which interact with the feed, their host and each other. This ecosystem is relatively poorly understood, particularly inter-species interactions and interactions with the host. Less than 15% of the microbial species in the gastrointestinal tract have been cultured and characterised. However, knowledge of this ecosystem is accumulating, particularly with the advent of molecular biology and culture independent technologies. New high throughput sequencing methodologies, such as pyrosequencing, will greatly improve the rate of knowledge acquisition and techniques such as Stable Isotope Probing will enhance our ability to understand species inter-relationships. While we can expect an increase in our knowledge of this complex ecosystem, and an improved ability to predictably lower CH4 emissions, examples of successful reductions already exist, including use of feeds (e.g., cereal grains) and chemical additives (e.g., 2-bromo-ethane sulfonate, bromochloromethane). Achieving meaningful reductions in CH4 emissions may be possible with advances in our knowledge of the intricacies of this complex ecosystem.