The combination of non-traditional stable isotope (Fe-Cu-Zn-Mo) theory and data has enhanced the understanding of redox processes in geological systems. This paper provides a comprehensive review of this relatively new field, including theoretical and experimental constraints on isotope fractionation behavior related to redox processes, oxygen fugacity at different spatial and temporal scales, and the use of isotopic tracers to study redox processes. Stable isotope theory predicts that Fe-Cu-Zn-Mo isotopes should respond to changes in redox states. The results indicate that Fe isotopes have promising applications as "oxybarometer" for magmatic processes, surface processes, and fluid properties in subduction zones. Cu isotopes can effectively trace redox processes in magmatic, hydrothermal, and terrestrial systems. Zn isotopes, due to their fractionation during complex chelation processes, have been used as sensitive tracers for the migration of sulfur/carbon-bearing fluids in various environments. Mo isotopes serve as paleo-oxybarometer and can be used to reconstruct the ancient ocean-atmosphere redox state effectively.