Three important classes of redox centers involved in electron transfer (ET) processes are reviewed. Together, however, they can cover the whole range, with cytochromes in the middle, Fe-S centers toward the lower end, and the cupredoxins toward the higher end. All three redox centers have structural features that make them unique, and yet they also show many similarities that make them excellent choices for ET processes. Because the redox-active iron is fixed into a rigid porphyrin that accounts for four of the iron's six coordination sites, most of the electronic structure and redox properties remain similar between different cytochromes. In completing the primary coordination sphere of the iron, cytochromes typically use a combination of nitrogen and sulfur ligations from histidine or methionine side chains, respectively; terminal amine ligation has also been observed. For iron-sulfur proteins, the reduction potential ranges are influenced to some extent by the number of irons because it affects the redox states and transitions.