In any photochemical reaction there is always at least one step where radiation (light) is part of the process. Generally, an activated step occurs upon absorption of radiation having the required energy. The value of the rate of energy absorption required to formulate the reaction rate can be obtained by means of a radiation balance. In photoreactor design this balance is usually something very different from a straightforward application of the well-known Lambert-Beer equation. The radiative transfer (photon transport) equation applied to participating and reactive, homogeneous and heterogeneous media is presented. It is derived from radiation energy transfer fundamentals. Expressions for the Local Volumetric Rate of Energy Absorption in homogeneous photoreactors, as well as in heterogeneous (photocatalytic) reactors are derived. In the latter, scattering effects are properly accounted for. The boundary condition for integrating the radiative transfer equation is obtained by formulating tubular radiation source (lamp) models of emission. Models for Voluminal and Superficial emission are presented. They are complemented with the description of a methodology for obtaining the usually complex limits of integration for the formal solution of the radiative transfer equation applied to photochemical reactors. Finally, a list of applications (reactor modeling with experimental verification) were these methodologies have been used is given. These examples, covering different reaction and reactor configurations show the utility and validity of this rigorous approach for photochemical reaction analysis and reactor analysis and design.