Archaea live under different environmental conditions, such as high salinity, extreme pHs and cold or hot temperatures. How energy is conserved under such harsh environmental conditions is a major question in cellular bioenergetics of archaea. The key enzymes in energy conservation are the archaeal A₁A_O ATP synthases, a class of ATP synthases distinct from the F₁F_O ATP synthase ATP synthase found in bacteria, mitochondria and chloroplasts and the V₁V_O ATPases of eukaryotes. A₁A_O ATP synthases have distinct structural features such as a collar-like structure, an extended central stalk, and two peripheral stalks possibly stabilizing the A₁A_O ATP synthase during rotation in ATP synthesis/hydrolysis at high temperatures as well as to provide the storage of transient elastic energy during ion-pumping and ATP synthesis/-hydrolysis. High resolution structures of individual subunits and subcomplexes have been obtained in recent years that shed new light on the function and mechanism of this unique class of ATP synthases. An outstanding feature of archaeal A₁A_O ATP synthases is their diversity in size of rotor subunits and the coupling ion used for ATP synthesis with H⁺, Na⁺ or even H⁺ and Na⁺ using enzymes. The evolution of the H⁺ binding site to a Na⁺ binding site and its implications for the energy metabolism and physiology of the cell are discussed.