Direct ultramicroscopic studies and measurements of the optical scattering spectrum of ``pure'' and doped alkali halide crystals are described showing the existence of elementary scattering centers and of assemblies of such centers. The elementary scatterers are established to be impurity precipitates of dimensions not exceeding a few hundred angstroms. The assemblies lead to characteristic ultra‐microscopically visible diffraction images representing decorations of branch points of a semi‐organized network of dislocations which reflect the state of strain and deformation in the subgrains of the mosaic structure of the single crystals. With increasing temperature the number of elementary scatterers decreases as impurities enter the host lattice into true solid solution, and the size of the assemblies diminishes.Electron spin resonance experiments on doped alkali halides are discussed which provide direct information on the process of dissolution and reprecipitation of impurity ions. Results on strong exchange effects in the electron spin resonance of the systems MnCl2&sngbnd;NaCl and MnCl2&sngbnd;KCl are presented which are explained in terms of the substructure of very small impurity precipitates deduced from the optical investigations.