Abstract The history of the earth's atmosphere has been divided into three stages. The first covers the earliest part of earth history, prior to the formation of the core; during this time native iron was probably present in the upper part of the mantle. Volcanic gases ejected during the first stage probably contained a large amount of hydrogen, and the atmosphere was highly reduced. Carbon was probably present in the form of methane, nitrogen may have been present as ammonia, and free oxygen was absent in all but nonequilibrium quantities. The second stage of atmospheric history began when iron was removed from the upper mantle. Volcanic gases became much less reducing and probably approachedrather rapidly the present oxidation state of Hawaiian volcanic gases. The atmosphere responded to this change largely in terms of the chemistry of atmospheric carbon. Carbon dioxide took the place of methane and began to participate in the processes of weathering and chemical precipitation. Free oxygen was probably absent during the second stage except in trace, nonequilibrium quantities. The third stage began when the rate of production of oxygen by photosynthesis exceeded the rate needed to oxidize injected volcanic gases completely. During this, the present, stage oxygen has probably accumulated at a roughly constant rate. The earth’s core was probably formed quite early, and the first stage of the atmosphere probably lasted less than 0.5 b.y. (billion years). The second stage may have continued until about 1.0 b.y. ago. The presence of detrital uraninite in mid-Precambrian sediments suggests that the second stage lasted at least until 2.0 b.y. ago, and the oxygen requirements of late Paleozoic insects suggest that a fairly high oxygen pressure obtained 0.3 b.y. ago.