In diffusion phenomena, such as the diffusion of gas molecules or of heat, it is found that the diffusing substance tends to move from regions of high density toward those of low density. Neutrons behave similarly, since there are more collisions per unit volume in regions of high density, and after colliding the neutrons move away from the collision centers. Thus neutrons tend to diffuse from regions of high neutron density to low neutron density. Fick’s first law of diffusion describes diffusion from high to low concentration as proportional to the particle’s concentration gradient, and were first posited by Adolf Fick in 1855 on the basis of his experimental results. In this chapter, we will introduce a Fick’s Law for neutrons, applied to the total flux, and discuss its validity.