AbstractAn overview is presented on the recent development and the new issues for mechanistic understanding of neutron radiation effects, placing emphasis on the temperature effects. The temperature dependence of the initial defect production and microstructural evolution is reviewed in the light of recent models and computer simulations. The varying temperature irradiation effect is pointed out to be an important new issue both for fundamental radiation effect studies and engineering-oriented materials development studies. The need for a materials irradiation facility and its role in the fundamental studies are discussed.