The relation between the current vector and the neutron flux is the Fick Law. Fick's law is true for big, weak absorbing medium consisting of heavy nuclei, it is far from local heterogeneity of medium and if the macroscopic cross section of scattering weakly depends on the space coordinate variables. Diffusion does not work with the hydrogen medium. The constant of proportionality is a new constant — the diffusion coefficient. One of the basic assumptions was that the potential scattering is isotropic in the laboratory frame. In fact for a nucleus, except the heaviest ones, it is not true. We will discuss this in the future. That means we need the transport correction. We introduce in neutron physics the new macroscopic cross section — the transport cross section equals to sigma total minus average cosine of the scattering angle multiplied by sigma S. Say, roughly, one collision of the neutron with a nucleus, which is anisotropic changed by a combined collision, which is isotropic. That is why the mean free path has increased. Fick's law means that the primary movement of the neutrons is directed from the higher neutron density place to the lower neutron place. Fick's law works under the conditions of the complete chaotic movement of neutrons in the medium (similarity to the Brownian motion of molecules). Where the chaos is broken (local heterogeneities, strong absorption, etc.) Fick's law does not work. By using the Fick’s Law we can get the one speed diffusion equation. There is a number of simplifications of the equation. We will consider stationary case and the medium is homogeneous which means that the diffusion coefficient and cross sections don’t depend on the space coordinates. And for a better understanding of how the neutron is distributed in space we often use a source of neutrons the only outer source without neutron multiplication by the fission. The diffusion coefficient can be pulled out of the divergence symbol, and the divergence of a gradient of function is a Laplacian applied to this function. Divide the last equation by the coefficient of diffusion and then, we will come to a new notion. The diffusion length is the ratio of the diffusion coefficient and the macroscopic cross section of absorption. The last equation is a differential equation of the second order regarding to the neutron flux function. Therefore, in its solution there are two constants which need to be defined from the physical problem description.