Martensitic Cr-alloyed high-temperature materials offer interesting opportunities for design and construction of advanced power plants. An extensive research programme has been carried out at the Research Centre of the Belgian Welding Institute and Laborelec on martensitic 12% Cr steel for gaining a better understanding of the failure mode and the deformation mechanism of welded joints under uniaxial and multiaxial loads. A large number of pipe girth welds were realized by three Belgian manufacturers (Cockerill Mechanical Industries, Fabricom and Mannesmann-Carnoy). Different filler metals were used and the influence of the welding regime (austenitic and martensitic), the post-weld heat treatment (PWHT) (single or double) and the base metal wall thickness on the high-temperature properties of the different weldments was evaluated. It was found that the creep properties of a 12% Cr weldment are not influenced by the welding regime and the base metal wall thickness. As would be expected, the creep strengths of the original 12% Cr base metal as well as the temperature of the PWHT have some effect. The existence of a typical failure in the intercritical zone (type IV region) is demonstrated and explained. The consequences for the design of welded 12% Cr components are indicated. More recently the research was extended towards improved 9% Cr steel (T91). A rather small preliminary programme for the orientation of further research showed a similar failure location as for 12% Cr steel, although the observed loss in strength of the weldment compared to the base metal tended to be considerably lower. The so-called‘half-tempering’treatment was tried out and the effect on the creep strength of the weldment is shown. A more fundamental national research programme on P91 steel has been established and is actually running.