CIP (complex initial permeability) spectra of PF (polycrystalline ferrite) are studied both as intrinsic and extrinsic properties. In the former case, main steps of modeling, based on effects coming from polycrystal grain sizes distribution and defects, are described. The obtained relations work well in practice for PF with more or less normal MS (microstructure) and no size effects. Besides, fundamental connection between parameters of CIP and MS is found. Another case—PF with possible size effects (MnZn-ferrites) are studied experimentally for different sizes of cores, unveiling the dependence of phenomena on: dimensions of cross-section, number of turns, width of nonmagnetic gap.