In the previous chapters, we discussed two special cases: diffusion without moderation (equation of diffusion for mono-kinetic neutrons in Chaps.  7 and 8 ) and moderation without diffusion (Chap.  4 ). In this chapter we shall calculate the space and energy distribution of neutrons during the slowing downSlowing down process, as a result of diffusionDiffusion. The study of this distribution is relevant since it determines the probability of loss by leakage. Our goal will be to obtain the slowing-down current \(q(\boldsymbol{r}, E)\) arising from given sources, in various geometries.