[en] The construction of nuclear power plants needs design codes (e.g. ASME or RCC-MR) to allow save and reliable design and operation. To meet the requirements of advanced future plants either existing codes have to be amended or new codes have to be developed. Future Generation IV plants like the very high temperature reactor (VHTR) or the sodium fast reactor (SFR) are currently in this stage of code development. Some important issues to be considered in this context are: design curves (lower bound scatter bands), creep-fatigue, negligible creep, crack growth (fatigue, creep, fast fracture), damage monitoring. The paper will provide some insight into the materials problems behind design rules. Examples will be taken from a martensitic steel (mod 9% Cr), oxide dispersion strengthened (ODS) steels and nickel-base superalloys. Possibilities for the replacement of the linear life fraction rule with advanced creep-fatigue damage assessments will be discussed. Specific emphasize will be put on the role of cyclic softening. Damage assessment and propagation of cracks from flaws under creep and fatigue will be touched upon. Some results from creep at low temperatures (irradiation creep, negligible creep) will be presented. (author)