  This review will deal with several types of free charge localisation in
oxides and their consequences on the effective dielectric spectra of such
materials. The first one is the polaronic localisation at the unit cell scale
on residual impurities in ferroelectric networks. The second one is the
collective localisation of free charge at macroscopic interfaces like surfaces,
electrodes and grain boundaries in ceramics. Polarons have been observed in
many oxide perovskites mostly when cations having several stable electronic
configurations are present. In manganites, the density of such polarons is so
high as to drive a net lattice of interacting polarons. On the other hand, in
ferroelectric materials like BaTiO3 and LiNbO3, the density of polarons is
usually very small but they can influence strongly the macroscopic
conductivity. The contribution of such polarons to the dielectric spectra of
ferroelectric materials is described. Even residual impurities as for example
Iron can induce well defined anomalies at very low temperatures. This is mostly
resulting from the interaction between localised polarons and the highly
polarisable ferroelectric network in which they are embedded. The case of such
residual polarons in SrTiO3 will be described in more details, emphasizing the
quantum polaron state at liquid helium temperatures. Recently, several
non-ferroelectric oxides have been shown to display giant effective dielectric
permittivity. It is first shown that the frequency/temperature behaviour of
such parameters is very similar in very different compounds (donor doped
BaTiO3, CaCu3Ti4O12, LuFe2O4,Li doped NiO,...). This similarity calls for a
common origin of the giant dielectric permittivity in these compounds. A space
charge localisation at macroscopic interfaces can be the key for such extremely
high dielectric permittivity.
