The time has arrived to deal with the most important topic of this book: the evolution of oxygen in the atmosphere of the Earth. There is a huge amount of material on this subject and we decided that the most reasonable reference is the works of Heinrich HollandHolland, Heinrich, James Kasting, and David CatlingCatling, David besides the classical approach of James WalkerWalker, James. It is quite clear that oxygen rise is related to the appearance of life on Earth but contrary to the popular belief the maintenance on the oxygen level is related to geochemistry and to the tectonic movements in the planet. The source of oxygen is clearly related to an imbalance between photosynthesis (which produces oxygen and organic matter) and respiration (which consumes oxygen) because part of the organic matter is subtracted to oxidation by burial. So it is clear that oxygen is related to the existence of life and to the cycling of carbon.