Effects of neutron irradiation on defect accumulation and physical and mechanical properties have been studied both experimentally and theoretically. Specimens of pure iron and RAFM (reduced activation ferritic-martensic) steels were irradiated todifferent dose levels and at different irradiation temperatures. The resulting microstructure was characterized using transmission electron microscopy, positron annihilation spectroscopy and electrical resistivity measurements. Mechanical properties weredetermined by uniaxial tensile testing. Dislocation-loop interaction, formation of rafts of loops, radiation hardening and formation of “cleared channels” were studied using different computational techniques. Experiments have shown that nano-voids areformed both in pure iron and F82H steel already at 50°C. In pure iron, the formation of nano-voids is detected already at a dose level of ~10-3 dpa. Also in iron, self-interstitial atoms were found to accumulate in the form of glissile and sessile loops;at higher dose levels, these loops led to formation of rafts of loops. Irradiation led to an increase in the yield strength, a sudden drop in the yield stress, and, at higher doses, the initiation of plastic instability immediately beyond the upper yieldpoint. Experimental as well as the results of computer simulations are found to be consistent with the cascade induced source hardening model.