I will review some recent developments about the 2D classical Ising model focusing on the method of Kac and Ward. In joint work with Daniel Ueltschi, we have extended this method to systems with negative coupling constants. As a consequence, we observe that the Cimasoni--Duminil-Copin--Li equation for the critical temperature, originally proved for general bi-periodic planar lattices with positive couplings, remains valid in a broader setting. Furthermore, I will discuss the possibility of extending this method to study the energy correlations for coupling constants of arbitrary sign. Finally, I will explain how this method can be used to rigorously compute the free energy of the 1D quantum Ising model.