High resolution X-ray diffraction measurements have been done on Si(001)-based structures grown by molecular beam epitaxy (MBE). By systematically varying the angle of incidence and the diffraction angle, the diffraction intensity data can be displayed in a two-dimensional X-ray diffraction intensity map that can be interpretted as a reciprocal space map of the reciprocal lattice points. The experimental technique is described and results for studies of the strain and the strain relaxation with high temperature annealing are described for the following material systems: strained Si/Sil xGex heterostructures, highly B-doped Si layers on Siand highly B-doped Sil_xGex layers on Si. The strain relaxation in Sil_xGex layers occurs via generation of misfit dislocations creating a shift and a characteristic mosaic broadening of the layer peak in the reciprocal space maps. A summary of how the degree of relaxation, as measured from reciprocal space maps, depends on the annealing temperature and the layer thickness is given. The relaxation of the strain induced by B, for doping concentrations up to 5 x 102Β°cm-3, is obtained by diffusion of B into the substrate. For a Si0.82Geo.ls layer with partially compensated compressive strain due to a B concentration of 3 x 102o cm-3, the maps show a combination of strain relaxation via misfit dislocations and B diffusion into the substrate.