A review of photoreactor engineering theory and applications, rigorously derived from chemical reaction engineering principles and radiative energy transport fundamentals, is described. The design procedure is illustrated by presenting the modeling of a continuous flow, annular reactor that is used in a predictive mode. In order to formulate the photochemical reaction rate properly, the radiative transfer equation applied to participating and reactive, homogeneous, and heterogeneous media is presented. We also describe the way by which the effects produced by reflecting surfaces may be incorporated into the rate of reaction and, consequently, into the reactor model. Likewise, a precise formulation to account for polychromatic irradiation is shown. The described methodology provides photoreactor mathematical models that permit an a priori design of a photochemical reactor. Thus, from laboratory data, the scale-up can be performed with no limitations in the size or shape of a large scale application