Past and current gas-cooled nuclear reactors rely on CO2-rich gases as a coolant to transfer heat from the reactor core, while future reactors may utilize a supercritical CO2 Brayton cycle for power production. The alloys used to construct these systems can experience serious corrosion issues induced by CO2 in the high temperature portions of the reactor and therefore alloy compatibility in these environments represents an important materials consideration. This article presents a summary of the extensive corrosion research that has accumulated for structural alloys in high temperature CO2 environments over a wide range of pressures relevant to past, current, and future nuclear reactors, with an emphasis on the Fe-Cr based steels that are most commonly used in these systems. The fundamental mechanisms controlling alloy degradation are presented and discussed in the context of the probable life-limiting processes to provide guidance and caution toward the informed selection of materials for future CO2 based systems.