Both thermal aging at 482 to 704°C for up to 25 000 h and high flux isotope reactor (HFIR) irradiation at 300 to 600°C for up to 39 dpa produce substantial changes in the as-tempered microstructure of 9Cr-1MoVNb martensitic/ferritic steel. However, the changes in the dislocation/subgrain boundary and the precipitate structures caused by thermal aging or neutron irradiation are quite different in nature. During thermal aging, the as-tempered lath/subgrain boundary and carbide precipitate structures remain stable below 650°C but coarsen and recover somewhat at 650 to 704°C. The formation of abundant intergranular Laves phase, intralath dislocation networks, and fine dispersions of vanadium carbide (VC) needles are thermal aging effects that are superimposed upon the as-tempered microstructure at 482 to 593°C. HFIR irradiation produces dense dispersions of very small “black-dot” dislocations loops at 300°C and produces helium bubbles and voids at 400°C. At 300 to 500°C, there is considerable recovery of the as-tempered lath/subgrain boundary structure and microstructural/microcompositional instability of the as-tempered carbide precipitates during irradiation. By contrast, the as-tempered microstructure remains essentially unchanged during irradiation at 600°C. Comparison of thermally aged with irradiated material suggests that the instabilities of the as-tempered lath/subgrain boundary and precipitate structures at lower irradiation temperatures are radiation-induced effects, whereas the absence of both Laves phase and fine VC needles during irradiation is a radiation-retarded thermal effect.