Studies on sulfur metabolism in archaea have revealed many novel enzymes and pathways and have advanced our understanding on metabolic processes, not only of the archaea, but of biology in general. A variety of dissimilatory sulfur metabolisms, i.e. reactions used for energy conservation, are found in archaea from both the Crenarchaeota and Euryarchaeota phyla. Although not yet fully characterized, major processes include aerobic elemental sulfur (S0) oxidation, anaerobic S0 reduction, anaerobic sulfate/sulfite reduction and anaerobic respiration of organic sulfur. Assimilatory sulfur metabolism, i.e. reactions used for biosynthesis of sulfur-containing compounds, also possesses some novel features. Cysteine biosynthesis in some archaea uses a unique tRNA-dependent pathway. Fe-S cluster biogenesis in many archaea differs from that in bacteria and eukaryotes and requires unidentified components. The eukaryotic ubiquitin system is conserved in archaea and involved in both protein degradation and biosynthesis of sulfur-containing cofactors. Lastly, specific pathways are utilized for the biosynthesis of coenzyme M and coenzyme B, the sulfur-containing cofactors required for methanogenesis.