Photocatalytic reactor design is a challenge which inherits the existing mass transfer issues of conventional catalysis with the addition of photon transfer issues. For more than three decades, many research teams have taken the challenge and various new design concepts have emerged to overcome the limitations. The majority of the research on photocatalytic reactor design was put to use on reactors with environmental applications, namely, wastewater treatment. This work is a review on the most important design concepts which had a high impact on the photocatalytic reactor designs. Another aspect covered in this work is the methods for comparing the wastewater treatment reactors. Several benchmarks have been considered and the new photocatalytic space–time yield benchmark measure has been demonstrated by comparing three different designs. With the aid of the new benchmark measure and the extensive state of the art, a new direction in the research on photocatalytic wastewater treatment is indicated, namely, lighting design instead of new geometries.@en