The fundamental aspects of photocatalytic reactor modeling are detailed in this chapter. The relevance of the radiation and scattering models applied to compound parabolic collectors (CPCs) and flat plate photoreactor (FPR) geometries with a kinetic model for different compounds is shown. The key factors of the modeling of a solar photoreactor and the optimization methods are explored from a new point of view. The mathematical structure of the model applied to the solar scale is described including its experimental validation.