The use of test reactors and hot cell facilities to study irradiated materials becomes more and more problematic. One way of partially solving this problem is to develop tools for computer simulation of radiation effects in materials, by applying multiscale modelling techniques. The development of such tools dedicated to the nuclear domain (called virtual test reactors: VTRs) began recently (1998), focusing on specific issues and using current simulation techniques. However, it is paramount to elaborate a new generation of techniques, which will allow the application field of VTRs to be enlarged within a few years. The objective of the ITEM Thematic Network was to ensure that these developments are performed rapidly and in a coordinated way in Europe. It was also a great opportunity for the European nuclear industry to lead such an international effort in the direction of its interest. Furthermore, the multiscale modelling approach is being aggressively pursued in the USA and in Japan. The network will allow Europe to exchange information at an equal level with these countries.