This paper points out the necessity of having adequate data on the high-temperature properties of metals used in steam-boiler construction. The significance of the time factor in testing in relationship to long-life high-performance steam-generating equipment is discussed as are the general effects of metal oxidation from steam and combustion atmospheres. Metallurgical stability of steels is important in maintaining creep strength and is necessary to prevent severe modification of mechanical properties through effects such as spheroidization of carbide phase, graphitization or formation of sigma phase in highly alloyed steels. The relative merits of long-time creep tests and stress-rupture tests are considered, and typical test data are cited for each form of test. These data are necessary for design of superheater tubes and headers, alloy baffles, hangers and fittings, and are an aid to the Code Committee in assigning suitable stress allowances for materials. The paper gives suggestions for temperature limits for superheater-tube materials and a résumé of field experience on carbon and alloy steels. Conclusions are drawn that high-temperature creep and rupture testing have been very useful in evaluating the relative strength of steels for high-temperature use, in aiding in the development of superior materials, and in permitting the increases in steam temperature and pressure which have led to much greater economy in power production. Further advances are forecast for the future through a well-co-ordinated program of metal testing now in progress.