Important information concerning atomic models of the defects in damaged crystals can be obtained from measurements of bulk length changes, &Dgr;L/L, and x‐ray lattice parameter changes, &Dgr;a/a, during irradiation and subsequent recovery. These quantities are not necessarily equal, and their magnitudes and signs may serve to discriminate between possible models. A detailed description of the &Dgr;L/Land &Dgr;a/aeffects to be expected from models consisting of point imperfections (either uniformly distributed or clustered), displacement spike regions, and dislocation loops is given. Additional discussion is given of the effects expected in measurements of low‐angle x‐ray scattering, Laue‐Bragg reflection broadening, Laue‐Bragg reflection intensities, and x‐ray diffuse scattering. The currently available experimental results on irradiated germanium are discussed in an attempt to discriminate between the various models. Comparatively simple models consisting of clusters of vacancies and clusters of interstitials are probably consistent with present experiment. No information is available regarding the shapes of these regions. More complicated models are not excluded, of course. The aforementioned techniques are compared with electrical measurements in semiconductors, and some of the strengths and weaknesses of the various methods are assessed. The need for further measurements of all types is emphasized.