[en] In conventional nuclear technology a lot of experience has been accumulated meanwhile for ferritic alloys, austenitic steels and Ni-based alloys as main component materials during R and D, design, construction and operation. Future Nuclear Power Plants such as Liquid Metal Reactors (LMRs), High Temperature Reactors (HTRs) or Fusion Reactors (FRs) are complex in the design and experiences with coolants/components are limited. Generally materials and their joining are key issues and their performance in components has not yet been demonstrated. Grades commonly used in fission reactors or conventional power systems have been proposed for the Gen IVs due to the extensive amount of work for the qualification and for code application. For high temperature operating conditions in heat exchangers of HTRs conventional alloys such as Alloy 800H or Alloy 617 have been specified in previous European projects (AVR, THTR, PNP,..). Both alloys have been tested already during the extensive German R and D activities for HTR/PNP projects between 1980-90. For the reactors of GEN IV, information gained from these activities and also from the European FBR -development (EFR) can be used and the available data may limit the extend of necessary R and D work. Design relevant properties especially of Alloy 617 from the previous R and D activities are summarized and compared with some recent results. The creep data from investigations for conventional power plants in the range of 700-800 deg. C fit well the data from the previous HTR programme. Problems such as reheat cracking are discussed. Critical issues of materials response to the helium environments are reviewed and the effect of ageing on mechanical properties. (author)