A nationally based program with the U.S. Department of Energy on in-reactor creep studies has produced experimental results that are being used for design and performance analyses of fast breeder reactors. These programs enabled the development of experimental methods that have produced copious in-reactor creep data over a broad range of conditions that include neutron fluences up to 1 X 1023 n/cm2 and temperatures as high as 750°C. These tests have revealed that contrary to guidance provided by post-irradiation creep tests, deformation during neutron irradiation does not impair the stress-to-rupture properties. Temperature- and stress-change experiments have been conducted to simulate the effects of nonsteady-state conditions existing in an operating reactor. The results show that the irradiation creep behavior is rather insensitive to stress and temperature history. In contrast, swelling is highly sensitive to temperature reductions that occur during irradiation. These studies have been conducted primarily on AISI Type 316 stainless steel although a broad base has been initiated on other alloys. The impact of these results is that irradiation creep at high temperature and high neutron fluences is larger than anticipated from early low fluence and low temperature data on AISI Type 316 stainless steel. Consequently, there is a high level of interest in advanced alloys that are more resistant to irradiation creep than is the AISI Type 316 stainless steel. Advanced alloys of the precipitation-strengthened nickel base class as well as low nickel ferritic steels are being investigated as alternates to AISI Type 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.