The critical Ising model is the critical point of the Ising model on a hypercubic lattice in two or three dimensions. It has a Z 2 global symmetry, corresponding to flipping all spins. The two-dimensional critical Ising model includes the M ( 4 , 3 ) Virasoro minimal model, which can be solved exactly. There is no Ising CFT in d ≥ ≥ 4 dimensions.