A general methodology for photoreactor analysis and design based on the fundamentals of chemical reaction engineering and radiative transfer in participating media is presented. Three applications in the field of advanced oxidation processes are considered to illustrate the proposed approach: (i) a photocatalytic reactor for air purification, (ii) a homogeneous photo-Fenton solar reactor, and (iii) a heterogeneous photocatalytic slurry reactor. In the first case, the procedure is exemplified with the modeling of a multiannular photocatalytic reactor for perchloroethylene removal from contaminated air streams. A rigorous physical and mathematical model of the multiannular concentric photoreactor was developed and experimentally verified. The second approach is illustrated with the degradation of a model pollutant by the Fenton and photo-Fenton reactions in a nonconcentrating, flat-plate solar reactor. Formic acid was chosen as the model substrate. The effect of the reaction temperature on the pollutant degradation rate is analyzed. In the case of the slurry photoreactor, the intrinsic kinetics of the photocatalytic decomposition of a toxic organic compound in aqueous solution, using suspended titanium dioxide catalytic particles and ultraviolet polychromatic radiation, is studied. The kinetic parameters are evaluated for different catalyst loadings, irradiation levels and pollutant initial concentrations. By means of these illustrative examples, the need of a systematic and rigorous approach to the analysis and design of photoreactors is emphasized.