A review of the microstructure of irradiated reduced activation ferritic/martensitic steels is presented, with a focus on F82H. Because of its resistance to irradiation induced hardening, swelling and low irradiation induced temperature shift in the ductile to brittle transition, this class of steels is a candidate structural material in future fusion reactors. The microstructure induced by irradiation is investigated in order to identify the key elements to deformation mechanisms. The study is focussed on F82H irradiated between 0.5 and 9.2 dpa at temperatures between 250 and 310 degreesC. Irradiation induced dislocation loops, when resolvable in the TEM, are known to have a Burgers vector a(0) , while for the smallest visible defects, or 'black spots,' there is still an uncertainty on their type. Black spot damage, possibly due to dislocation loops or precipitates, is investigated. It is attempted to quantitatively relate the dislocation loops and the precipitates to the irradiation induced hardening. (C) 2002 Elsevier Science B.V. All rights reserved.