Polarons have been observed in many insulating oxides, and also in the insulating phase of High Temperature Superconductors materials (HTSC). The latter are highly polarizable and can be strongly doped, leading to the Metal-Insulator Transition (MIT). The formation of polarons and their mutual interactions cannot be forgotten as an essential feature of the MIT. Due to strong coupling, polarons still form at rather high doping levels in the insulating phase and, due to their high effective mass, may cristallize in a polaron Wigner lattice. The aim of the project is to investigate different theoretical scenarii for the MIT, depending on the electron effective mass, polarizability and phonon frequencies. On may obtain a gas of polarons, or a quasi-free carrier gas, possibly coexisting with localized polarons. This is aimed to clarifiy the nature of the metallic phase and the relevance of polaron formation in the superconductivity mechanism.