Several fundamental aspects of defect cluster accumulation in irradiated ceramics and face centered cubic metals are reviewed, with particular emphasis on radiation hardening and the microstructural evolution in Cu observed by transmission electron microscopy (TEM). At temperatures where interstitials are mobile (> 50 K in Cu), the defect cluster density in pure Cu is initially proportional to the dose and exhibits a square root dose dependence above ∼ 10−4displacements per atom. This fluence dependence (determined from electrical resistivity and TEM studies) helps to resolve a long-standing controversy on the fluence dependence of radiation hardening. The fraction of freely migrating interstitials in copper irradiated with 14 MeV neutrons at room temperature is ∼ 11%. The activation energy for annealing stage V (stacking fault tetrahedra evaporation) in neutron-irradiated copper has been measured to be 0.84eV. Some features of the point defect accumulation behavior in ceramics are found to be very similar to the trends observed in pure Cu, despite the obvious differences in the physical properties of these two types of materials. Finally, microstructural evidence for some processes unique to nonmetals are summarized, particularly ionization induced diffusion.