Iron oxide minerals have been the focus of many review articles, books, and book chapters because of their widespread occurrence on Earth in extended ranges of thermodynamic and biogeochemical conditions. The environmental dependence of iron oxide formation, itself, enhances the relevance of studying iron oxides as indicators of present and past conditions (e.g., temperature, moisture, pH, and redox state). In this brief chapter we give a succinct and selective view of some key aspects of iron oxide mineral formation and transformations and their relationship with geological and environmental conditions; for much more comprehensive reviews, the reader is referred to the works listed above. We begin with an abridged summary of the origins and distribution of terrestrial iron, followed by a classification and cataloging of iron oxide minerals. The final sections cover processes and conditions of formation and transformation of these minerals, which we highlight in more detail for one particular occurrence, the loess/paleosol sequences in which iron oxides have proven to be extremely valuable paleoenvironmental indicators.