Methane is the second most important greenhouse gas in the atmosphere with a concentration (rate by volume) of 1.7 x 10(-6). Approximately 75% of methane in atmosphere comes from the activity of methanogens in the worldwide. The methanogens are anaerobic archaebacteria, according to 16S rRNA and other biochemical taxonomy. The methanogens can use compounds such as acetic acid, hydrogen and CO2 athylamine as substrates for methane production. The biochemical metabolisms of methanogens are greatly affected by natural environment where they live in. This paper will present a general situation of methanogens in term of history, morphology, taxonomy, metabolisms and ecology.