Chromium-molybdenum martensitic (ferritic) steels such as 9 Cr-1 Mo-V-Nb and 12 Cr-1 Mo-V-W are candidates for fast reactor and fusion reactor applications. In a fast reactor, the effect of neutron irradiation is caused by displacement damage, that is, by the interstitials and vacancies that are created by the high-energy neutrons. Increases in strength occur for irradiation up to ∼450°C. This hardening is largely attributed to the dislocation loops that form from the agglomeration of the interstitials. Precipitates that form during irradiation can also contribute to the hardening. At higher temperatures, most of the displacement damage anneals out. Irradiation effects expected in the first wall of a fusion reactor differ from those in a fast reactor. In addition to displacement damage, large amounts of transmutation helium will also be produced. The simultaneous effects of displacement damage and helium can be simulated by irradiating nickel-doped ferritic steels in a mixed-spectrum fission reactor. Helium is produced by transmutation reactions between thermal neutrons and nickel, and displacement damage is formed by the fast neutrons of the spectrum. Results using this technique indicate that hardening occurs as in a fast reactor, but the helium causes a strength increase in addition to that caused by displacement damage alone. This effect of helium could have a significant effect on other properties, especially toughness, and must be considered in the design of fusion reactors.