Iron is the most abundant element on Earth that undergoes reduction-oxidation (redox) reactions. Moreover, the ratio of the iron isotopesFe andFe can change during these redox reactions. As Johnson and Beard discuss in their Perspective, this isotope fractionation has attracted interest as a way of understanding important biogeochemical processes on Earth over time. This isotope system may help unravel some of the questions surrounding changes in atmospheric oxygen and the origin and evolution of life over the past 4 billion years.