The evolution of the Earth's atmosphere is essentially the story of atmospheric oxygen. Virtually every realm of the Earth sciences-biology, geology, geochemistry, oceanography and atmospheric science-is needed to piece together an understanding of the history of oxygen. Over the past decade, new data from these fields has shown that there were two significant increases in atmospheric O2 levels at around 2.4-2.3 and 0.8-0.6 billion years ago, respectively. Throughout Earth history, oceanic sulfate concentrations appear to have increased in accord with greater O2 levels, while levels of methane, a strong greenhouse gas, may have inversely mirrored O2. Both oxic transitions occurred in eras characterized by "Snowball Earth" events and significant disturbances in the carbon cycle, perhaps associated with increases in O2 and losses of methane. To understand what controlled the oxygenation of the atmosphere, it is necessary to determine how O2 is consumed on geologic time scales through reaction with reductants released from the Earth's crust and mantle. There was apparently a long delay between the appearance of oxygenic photosynthesis and oxygenation of the atmosphere, and a plausible explanation is that excess reductants scavenged photosynthetic O2 from the early atmosphere. However, a quantitative understanding of how and why O2 became abundant on our reducing planet is still lacking. Thus, the study of the early atmosphere remains a frontier field with much to be discovered.