Iron oxides are ubiquitous in Earth and their redox reactions control the cycling and availability of important elements. In my dissertation, we explored the connections between mineral surface defects and iron redox chemistry. Specifically, we investigated how the presence of surface defects influences Fe(II)-Fe(III) electron transfer and microbial reduction of goethite. Our results provided new insights into the redox mechanism, allowing us to make better predictions about the behavior of iron oxides in natural environments.