As in conventional high temperature technology, the qualification of metallic materials for high temperature reactor (HTR) applications is based on creep behavior, fatigue properties, corrosion resistance, and thermal stability. Of specific interest are the effects of the primary coolant helium, which contains trace impurities of hydrogen, methane, carbon monoxide, and water vapor, on mechanical behavior. In addition, irradiation effects on the properties of absorber rod cladding and tritium permeation from the primary coolant into the process gas are important areas for investigation. The results show that, for test times of up to 20 000 h, the creep-rupture strength in air and in HTR helium lies in the same scatter band. The results of low cycle fatigue tests indicate a beneficial effect of HTR helium on the cycles of failure. Investigations of corrosion in HTR helium have shown that acceptable corrosion resistance can be achieved by strict control of the impurity content of the helium. Using the available creep-rupture data and the linear damage accumulation rule, the acceptable service lives of intermediate heat exchanger tubes were calculated for Inconel alloy 617 at 950°C. The data that are being accumulated from the various test programs will form the basis of a design code for nuclear components operating at temperatures >800 °C.