Agriculture, both in Slovenia and globally, significantly contributes to methane emissions. Methane is produced in the rumen in a process called hydrogenotrophic methanogenesis, performed by methanogenic archaea. They utilize H2 and CO2 to form CH4. In the last decades, researchers have published many studies focused on methanogenesis and methanogenic archaea, foremost due to CH4 emissions from ruminants. These contribute substantially to global CH4 emission and thus to the greenhouse effect. Furthermore, the methanogenesis and the eructation of the CH4 represent a loss of feed energy for the host animal. Several genera and species of the methanogenic archaea are known. Most of them belong to the family Methanobacteriaceae. Some live freely in the rumen fluid while other participate in symbiosis with rumen protozoa or fungi. Several ways are known for the mitigation of ruminal methane production. These include either inhibition of H2-producing reactions or promotion of H2-utilizing reactions. Both can be achieved by addition of various feed additives to the feed, with defaunation, with vaccination against methanogenic archaea, etc. The most effective way for reducing methanogenesis is the addition of feed antibiotics, which are banned in EU since 2006, however this is the reason for the extensive search for possible alternatives.