Abstract Fusion reactor design to an integrated first wall loading of 40 MW/y/m 2 or 500 dpa, produces extreme demands on structural materials. It is very difficult to conceive of material behavior at such high damage level. In an effort to assess the consequences of such damage, a series of commercial alloy specimens have been examined following irradiation with fast neutrons to doses on the order of 100 dpa (20% of goal fluence) at 400°C, 425°C or 540°C. The alloys were iron or nickel based and included ferritic (martensitic) and austenitic alloys. Examinations incorporated transmission electron microscopy and analytical electron microscopy of irradiated specimens. Microstructural examination confirm that ferritic alloys are very low swelling, but precipitate development can be very complex. Austenitic alloys can be very high swelling but no clearly defined microstructural differences were found between two alloys of similar composition but very different swelling response or between alloys of very different composition but with similar levels of swelling. Differences are ascribed to differences in the onset of swelling.