I have been involved also in a study of electronic correlation effects in crystals using Toyazawa's electronic polaron method. As a result, the Excitonic Hamiltonian Approach (EHA) has been proposed in [B14,B21]. In this method, crystal excitations are described in terms of Frenkel's localised excitons in the excitonic-operator representation. Then, the corresponding Hamiltonian is rigorously derived to bring one-to-one correspondence between the excitonic and usual second-quantisation electron-hole representations. This theory has been applied in [B14] to alkali halide crystals, where dispersion curves of the free excitons and interaction energy of two probe charges as a function of the distance between them have been calculated. In [B21] an attempt has been made to apply the EHA for studying polarisation effects in disordered systems. It was also shown how these ideas can be employed in the description of more fundamental crystal properties such as charge-current correlation functions (unpublished). The EHA has been also used in numerical calculations of the self-trapped hole localisation energies in KCl and corundum crystals [B33,B35,B36,B46].