The application of x‐ray diffraction microscopy to the study and control of crystal perfection of silicon is discussed. The major significance of this technique in establishing standards by which the validity of other methods, such as etch‐pit counting and decoration techniques, can be gauged is pointed out. The method previously used only for the mapping of dislocations in single crystals is now applied for the detection of impurities, segregation effects, cluster formation, and second phases. Segregation effects are studied in crystals containing oxygen. It is shown that, due to microsegregation of oxygen, the silicon lattice can become strongly anisotropic. The influence of such a lattice on the diffusion properties is shown. Precipitation effects are investigated in crystals containing copper precipitates. The results are verified by infrared microscopy. Experimental details of a method unique for the investigation of large crystal wafers are given.