Vacancy diffusion is caused by an empty space in a crystal lattice. Interstitial diffusion is caused by the presence of a different type of atom that may be present in the material.Diffusion is the migration of atoms from a region of high concentration to a region of low concentration. In a homogeneous material, atoms are routinely moving around but the movement is random (i.e. there is always an equal number of atoms moving in all directions). In an inhomogeneous material, all the atoms are moving near randomly, but there is a migration of atoms to areas where their concentrations are lower. In other words, there is a net diffusion.

Atom diffusion can occur by the motion of host or substitutional atoms to vacancies (vacancy diffusion), or interstitial impurities atoms to different interstitial positions (interstitial diffusion). In order to move, an atom must overcome the bond energy due to nearby atoms. This is more easily achieved at high temperatures when the atoms are vibrating strongly. Carburizing, which will be discussed later, is an example of diffusion is used.