Archaea are a collection of unicellular, organelle-free organisms once included among prokaryotes but now assigned to their own domain. The domain is the highest division of taxonomic classification, one level above the kingdom.

Like prokaryotes, Archaea lack nuclei or any other membraned organelles, and they are exclusively unicellular. However, despite their simplicity, Archaea have been found living in a huge range of different environments on Earth, including extremophiles, which get their name from living in conditions as volatile as deep sea vents, volcanic springs, and highly acidic or alkaline soils. There are even Archaea present in a wide range of organisms (like humans), where they aid symbiotically in digestion.

Comparatively little is still understood about Archaea, but because they are so diverse and often have properties that separate them from the other two domains, much hope lies in applications of Archaea, from engineering them to extract metals to creating modern antibiotics from them in the fight against resistant strains of bacteria.