Chapter 2 starts with a brief review on the phase diagram of high-Tc cuprates, particularly on the phases of Mott insulators and pseudogaps. A number of microscopic models of high-Tc superconductors, including the three-band Hubbard model and its effective low-energy models in the strong coupling limit, namely the t-J model or its equivalent single-band Hubbard model, are then introduced. The models for describing the interlayer hopping and the system with Zn or Ni impurities in the copper oxides are also discussed. The Friedel sum rule is shown to be severely modified in the strong coupling limit, which reveals the perplexing but inherent nature of Zn as a unitary scattering potential of non-magnetic impurity.