This chapter describe the methodology for the full predictive simulation of photocatalytic reactors with engineering and design purposes. A rigorous approach based on the resolution of the conservation equations of momentum, mass and radiant energy of the system will be used, emphasizing the similarities and differences with conventional chemical reactors. One of the key steps for making feasible the predictive simulation of reactors with different sizes, geometries and light sources is the availability of an intrinsic kinetic model of the process based on a reaction mechanism that takes into account the radiation absorption step in an explicit way. The procedure for the determination of this type of kinetic model based on experimental data at laboratory scale requires knowledge of the optical properties of the photocatalytic material, requisite for the resolution of the radiative transfer equation that would provide the rigorous description of the radiation field in the reactor. A case study of the application of this methodology to the photocatalytic oxidation of cyanide with TiO₂/SiO₂ materials is included, describing in detail each step of the procedure.