We review the recently introduced concept of exact mean-field theories for fermionic lattice models. The method is applied to the Extended Simplified Hubbard Model, which is an Extended Hubbard model (including nearest-neighbor interaction) where only the ↓-electrons can hop. The construction of an exact mean-field Hamiltonian is discussed and a summary is given of the results obtained for the critical temperature, the order parameter and the density of states.