Book excerpt: The volume contains the lectures delivered at the "Fifth Training Course in the Physics of Correlated Electron Systems and High-Tc Superconductors". The lectures are up-to-date comprehensive overviews of relevant subjects in condensed matter physics. Contributions include: the extension of the BCS theory to the strong-coupling regime with small polarons and bipolarons, the canonical Migdal-Eliashberg theory of strongly coupled electrons and phonons (or any bosons) and its breakdown in the strong-coupling regime, the multi-polaron problem, the bipolaron theory to cuprates, the essential interaction in oxides, the parameter-free expression for the superconducting critical temperature, upper crtitical field, symmetry of the order parameter, the London penetration depth, vortex structure, theory of tunnelling and Andreev reflection, normal and superconducting gaps, angle-resolved photoemission, and stripes, the principles of the type II superconductor behavior, the magnetic field dependence of pinning mechanisms in Bi2Sr2CaCu2Ox thin films, the melting of the vortex lattice in Nb/CuMn multilayers, the transport properties of high-temperature superconductor-based superlattices, the theory of strongly correlated low-dimensional systems, microscopical theory of spin-superconducting pairing in systems with strong electron correlations, p-d model, Hubbard and t-J models, retardation effects, antiferromagnetic superexchange interaction, the non-perturbative techniques in (1+1) and two dimensions, the bosonization, the WZNW models.