A review of the Magnox Reactor experience shows the progressive development of the steam generator with increasing power density giving rise to more severe restrictions in fabrication of the steam generator elements into the boiler matrix. This has given rise to a steadily increasing demand for mechanized fabrication techniques to ensure adequate precision during manufacture. Successful operation of the once-through system in conjunction with the annular disposition of the boiler units in the prestressed concrete pressure vessel made the Oldbury A Magnox station a natural basis for development of the Advanced Gas Cooled Reactor (AGR) system. The principal criteria in selection of material are the higher temperature and the more aggressive CO2 coolant which contrasts with the need to avoid evaporation in austenitic material. Breakaway corrosion has limited carbon steel to a maximum of 350°C, while a minimum superheat of 50°C is considered necessary to avoid stress corrosion cracking in the austenitic section of a once-through boiler required by the AGR maximum gas temperatures of 634°C. Evidence of potential breakaway corrosion in 9% Cr—1% Mo at 550°C and above has caused concern for long-term operation at lower temperatures and has resulted in the maximum metal temperature being reduced to 450°C with resulting narrower margins on the degree of superheat. This has resulted in an extensive stress corrosion program aimed at defining the risk parameters prior to commissioning the first AGR station. Identification of fretting phenomena associated with the design of Hinkley B steam generator supports resulted in a welded support design and an extensive substantiation program on the integrity of the support system. The practical implication of the material chosen in the design and the manufacture of the steam generators for the Hinkley Point B and Hunterston B plants have emphasized the importance attached to quality control, typically during manipulation of bends.