The Chap.  13 highlights that the one group modelization cannot properly calculate a reflected reactor, whether in terms of critical condition or shape of flux. This justifies the need to take into account of changes in neutron energy during the slowing down process. Thus, the energy-dependent diffusion equation is commonly transformed into a set of coupled equations, in the energy groups resulting from the discretization of the neutron energy domain. We provide a simplified approach to the neutron transfer from one group to the underneath group. Most of industrial codes for core calculation in the world rely on two neutron energy group model for the calculation of PWRs (However in the case of BWRs more groups are needed). Thus, in this chapter, we establish the two neutron energy group set of diffusion equations.