This chapter introduces the concept of the radiation distribution factor, defined as the fraction of power emitted from one surface of an enclosure that is absorbed by another surface due to direct radiation and to all possible reflections. The distribution factor can be thought of as the sensitivity of the heat absorbed by a given surface to the heat emitted from any other surface. Thus, it can be used not only to compute the distribution of heat throughout an enclosure due to radiation, but also as a design tool targeting selected surfaces to determine their relative exposure to radiative heating. We consider enclosures composed of diffuse-specular gray surfaces; firstly, because of their pedagogical importance and also because of their relevance in preliminary design.