Archaea are prokaryotes that represent a unique evolutionary lineage distinct from bacteria and eukaryotes. Once thought to exist only in extreme environments, archaea inhabit a wide range of habitats, including soil, marine environments, and the digestive systems of living organisms. This section highlights the systematic diversity of archaea, particularly the superphyla Euryarchaeota, TACK, DPANN, and Asgard, along with their respective phyla. Additionally, the ecological distribution of archaea, their morphological and structural differences, their extensive metabolic properties, and their adaptation mechanisms in extreme conditions are discussed to highlight their biological importance and contribute to our understanding of life on our planet.