▪ Abstract This paper reviews the Precambrian history of atmospheric oxygen, beginning with a brief discussion of the possible nature and magnitude of life before the evolution of oxygenic photosynthesis. This is followed by a summary of the various lines of evidence constraining oxygen levels through time, resulting in a suggested history of atmospheric oxygen concentrations. Also reviewed are the various processes regulating oxygen concentrations, and several models of Precambrian oxygen evolution are presented. A sparse geologic record, combined with uncertainties as to its interpretation, yields only a fragmentary and imprecise reading of atmospheric oxygen evolution. Nevertheless, oxygen levels have increased through time, but not monotonically, with major and fascinating swings to both lower and higher levels.