A rigorous approach developed in the past twenty years for photoreactor analysis and design is presented. It has been derived from multicomponent mass and energy transport fundamentals. It comprises the appropriate use of the radiative transport equation applied to participating and reacting media. The objective of the proposed methodology is to provide photoreactor mathematical models that permit an a priori design of a photochemical reactor without resorting to empirically adjusted parameters or costly pilot plant experiments. Thus, from laboratory data the scale-up can be performed with no limitations in the size or shape of a large scale application. Illustration of the procedure is described by presenting in details the modeling of a continuous flow, annular photoreactor where the non-isothermal photochlorination of methane in the gas phase is carried out under practical operating conditions. Particular emphasis is put into the treatment of the rate of initiation that is the distinct characteristic of a photochemical process. The paper also presents the way by which the effects of the presence of reflecting surfaces can be incorporated into the rate of initiation and, consequently, into the reactor model. Likewise a precise way to account for polychromatic irradiation of practical reactors is described. References to previous research works are made throughout the paper indicating the key theoretical and experimental works that have been used to validate the proposed design method.