Only for the past 4 decades, archaea have been recognised as a separate domain of life, being classified as bacteria beforehand. Our initial findings have exclusively relied upon cultivatable extremophilic representatives of archaea, up until the development of more sophisticated molecular methods for surveying environmental genomes. Their use made apparent that archaea appear in all environments and in substantial quantities. In the recent decades we have most expanded our knowledge about their genetic diversity and their similarities with the domain of eukaryotes. The monophyly of all groups if still a subject of research and, very often, deeper branchings or new phyla are being found. Physiologically, archaea possess bacterial-like characteristics, eukaryote-like characteristics and distinctly archaeal properties. Furthermore, they are more alike eukaryotes than bacteria on the molecular scale. The discovery of the Asgard superphylum has brought us closer to understanding events that happened during eukaryogenesis. Following that notion, it has been established that eukaryotes are a branch of the archaeal domain, as descendants of Asgard archaea and bacterial endosymbionts. The full extent of eukaryotic properties which we ascribe to an archaeal ancestor is a subject of many theories. They will only be resolved by further expansion of phylogenetic diversity and future archaeal culturing.