[en] Creep studies have produced experimental results which are being used for design and performance analyses of fast breeder reactors. These programs enabled the development of experimental methods which have produced in-reactor creep data over a broad range of conditions which include neutron fluences up to 1 x 10²³ n/cm² and temperature as high as 750⁰C. These tests revealed that contrary to guidance provided by postirradiation creep tests, deformation during neutron irradiation does not impair the stress-to-rupture properties. Temperature- and stress-change experiments conducted to simulate the effects of non-steady-state conditions existing in an operating reactor show that the irradiation creep behavior is rather insensitive to stress and temperature history. In contrast, swelling is highly sensitive to temperature reductions which occur during irradiation. These studies were conducted primarily on AISI 316 stainless steel. Studies have been initiated on other alloys. These results show irradiation creep at high temperature and high neutron fluences is larger than anticipated from early low fluence and low temperature data on AISI 316 stainless steel. Advanced alloys of the precipitation-strengthened nickel-base as well as low nickel ferritic steels are being investigated as alternates to AISI 316 stainless steel for specific core applications. A diverse range in resistance to in-reactor creep has been found, with Inconel 706 providing very high resistance and PE16 providing an intermediate level of resistance