Methanogenesis is an anaerobic metabolism responsible for the generation of >90% of the methane formed on Earth today, with important implications for fuels production and global warming. Although methanogenic Archaea have been cultured for over 70 years, key insights regarding electron flow and energy conservation in methanogenesis have only recently emerged. Fundamental differences between two metabolic types of methanogenesis, hydrogenotrophic and methylotrophic, are now understood, with implications for metabolic versatility and the potential for engineering of methanogens to utilize new substrates. The development of model species with genetic and bioinformatic tools has advanced the field and holds potential for further characterizing and engineering of methanogenesis. Our understanding of a related pathway, anaerobic methane oxidation, is in its infancy.