Molecular oxygen appeared in the atmosphere about three billion years ago, due to the oxygenic photosynthesis invented by the ancestors of the cyanobacteria. Oxygen forms “reactive oxygen species”, which are highly toxic. To deal with the threat nature developed two membrane integrated enzymatic systems which reduce oxygen to water and, at the same time, use the enormous energy of this reaction to produce biologically important energy carriers. These enzymes are the proton pumping haem-copper terminal oxidases, e.g. cytochrome c oxidase, and the bd oxidases. The atomic structures of representative members of both enzyme families will be presented, and their mechanisms of action will be discussed. These evolutionary unrelated enzymes apparently use the same mechanisms to conserve energy and to prevent the formation of toxic reactive oxygen species, although the bd oxidases do not pump protons across membranes.