In agreement with their close relationship to eukaryotes, Archaea encode informational processing machineries that closely resemble those of eukaryotic representatives. The composition of archaeal cell membranes differs fundamentally from those of Bacteria and eukaryotes. This chapter provides an overview of additional lineages affiliating with the Euryarchaeota, including methanogenic lineages that have been extensively studied in the past. The Lokiarchaeota represents an archaeal lineage originally referred to as the Deep Sea Archaeal Group or Marine Benthic Group-B archaea, which are abundant in diverse marine sediments. Methanogenic archaea are part of the oral archaeome with Methanobrevibacter oralis being the most frequently detected species. Nonetheless, antibiotic-resistant archaea may become indirectly susceptible to antimicrobial treatments when relying on chemically susceptible bacterial partners within their complex communities. Thought to be of limited ecological relevance originally, Archaea are now known to inhabit a wide range of ecosystems and to play a key role in major biogeochemical cycles.