This lecture covers the diffusion of neutrons, focusing on the method of eigenfunctions for distributed sources and the modeling of elastic collisions between neutrons and target nuclei. It also delves into the solution for flux in a finite medium using eigenfunctions. Typical values of diffusion and length are discussed, along with the neutron slowing down process and elastic collisions. The lecture concludes with a summary of the diffusion area and length, emphasizing the analytical solutions of the diffusion equation.