Redox reactions of hemoglobin have gained importance because of the general interest of the role of oxidative stress in diseases and the possible role of red blood cells in oxidative stress. Although electron paramagnetic resonance (EPR) is extremely valuable in studying hemoglobin redox reactions it has not been adequately used. We have focused in this review on the important contributions of EPR to our understanding of hemoglobin redox reactions. We have limited our discussion to the redox reactions thought to occur under physiological conditions. This includes autoxidation as well as the reactions of hydrogen peroxide generated by superoxide dismutation. We have also discussed redox reactions associated with nitric oxide produced in the circulation. We have pinpointed the value of using EPR to detect and study the paramagnetic species and free radicals formed during these reactions. We have shown how EPR not only identifies the paramagnetic species formed but can also be used to provide insights into the mechanism involved in the redox reactions.