Irradiation is known to lead to degradation of mechanical properties of materials. Structural materials for nuclear applications can be affected by helium produced by transmutations. The objectives of this work were: (a) to investigate the effects of He on the mechanical properties of steels in different irradiation conditions (neutron spallation source and He-implantation), (b) to study the irradiation-induced hardening effect and the contribution of He on the hardening, (c) to compare the barrier strength of helium bubbles to moving dislocations as well as that of defect clusters and (d) to interpret the individual and combined barrier strengths to the overall hardening contribution. These goals have been reached by performing detailed hardness measurements on F82H ferritic/martensitic (f/m) steel after spallation neutron irradiation to 6.2-20.3 dpa (displacement per atom) at 151-344 °C and to a He-concentration of 465-1825 appm (atomic part per million) as well as after annealing treatments (300 °C/1 h, 400 °C/1 h, 600 °C/1 h and 600 °C/200 h). The annealing treatments were applied to remove the defect clusters stepwise and make the He-bubbles grow. Austenitic steel SS316L and f/m steels F82H and Optifer, helium implanted (0.2 dpa , 1350 appm He) with a 28 MeV He-beam at room temperature and after annealing were used to gain complementary information on the barrier strength of the helium bubbles. A significant increase in hardness following irradiation and implantation was observed: the largest increase in hardening was observed for the specimen with the highest received dose and He concentration. In comparison to the as-irradiated specimens, the hardness values of the annealed specimens were distinctively lower. The annealing demonstrated a continuously decrease with increasing annealing temperature. Detailed transmission electron microscopy (TEM) has been performed on STIP-II irradiated specimens in the as-irradiated condition and after annealing at 600 °C/1 h and 600 °C/200 h as well as on He-implanted specimens in their annealed condition. These investigations provided quantitative information on size distribution and density of defect clusters and helium bubbles as a function of irradiation dose, helium concentration and irradiation temperature. The helium bubbles were found to be weaker obstacles to dislocation than the defect clusters whereas their contribution to irradiation hardening cannot be neglected owing to their high density. Scanning transmission electron microscopy in combination with electron energy loss spectroscopy was performed on selected specimens. These investigations provided quantitative information on the helium number density.