The prototype nuclear process heat plant and the high-temperature gas-cooled reactor need materials that can withstand temperatures up to 1223K (950°C). An elaboration of fracture mechanics concepts that holds for the complete temperature regime must consider all possible phenomena like creep damage and precipitation during exposure, etc. In tests on the Inconel-617, Hastelloy-X, and Nimonic-86 alloys with respect to fatigue crack growth, creep crack growth, and toughness (J integral R curves) up to 1273 K (1000°C), the first creep crack growth results were obtained in helium to compare with the air results. It was shown that pure fatigue crack growth behavior can be described by linear elastic fracture mechanics up to 1273 K. An example of Hastelloy-X at 1223 K proves that evaluating fatigue crack growth according to the J integral concept gives, within a small scatterband, the same results as by following the linear elastic concept. Hastelloy-X shows a decreasing fracture toughness with increasing temperatures. It is emphasized that the J integral concept holds only if creep deformation can be neglected. The experimental evidence at highest temperatures shows that the J integral R curve is not at all similar to that found at lower temperatures under ideal conditions. Creep crack growth for Nimonic-86 at 1073 ≤ T/K ≤ 1273 shows that crack growth at 1223 K in helium is found to be larger than in air. Problems arise when correlating the creep crack growth results. The application of the energy rate integral C* seems promising, but this has yet to be proven. A combination of long-term creep with fatigue crack growth is presently impossible.