This overview introduces the contributions in this Special Issue with the aim of presenting an integrated picture of it. The contributions cover several important areas: protein stability and function under extreme conditions, osmotic stress and osmoadaptation, the structural features of the cell membrane and their possible significance with regard to heat stress, the molecular chaperone machine and multicellular structures as anti-stress mechanisms, peptidyl-prolyl cis-trans isomerases, proteases and the proteasome, and oxidative stress and the role of superoxide dismutase. These topics are briefly discussed to explain the basic concepts underpinning them, quoting for the most part introductory articles or reviews that might help the non-specialist to become familiar with the central themes of the Special Issue. As mentioned in the Preface every effort has been made to discuss the archaeal features within the context of other disciplines and biology in general, against the background of what is known for bacteria and eucarya. Hopefully, this approach will help the reader in understanding what is unique to the archaea, what is shared between them and the members of the other two phylogenetic domains, and how studies in archaea impact on other fields of science.