The purpose of these notes is to describe some of the physical and mathematical properties of the equations occurring in global illumination. We first examine the physical assumptions that make the particle model of light an appropriate paradigm for computer graphics and then derive a balance equation for photons. In doing this we establish connections with the field of em radiative transfer and its more abstract counterpart, transport theory. The resulting balance equation, known as the equation of transfer, accounts for large-scale interaction of light with participating media as well as complex reflecting surfaces. Under various simplifying assumptions the equation of transfer reduces to more conventional equations encountered in global illumination.