Heptazine-based polymeric carbon nitride (PCN) materials stand out from a range of other typical, mostly metal oxide-based photocatalysts mainly by their unique optical (i.e., bandgap and intra-gap states) and photoredox (i.e., quasi-Fermi levels of electrons and holes) properties. Nevertheless, the typical absorption onset of PCN is around 430-450 nm, which is far from making it an ideal absorber of solar radiation. This motivated significant research efforts directed towards shifting the optical absorption and the corresponding photocatalytic activity to the red. In general, one can distinguish two basic strategies for achieving this goal. Firstly, modified synthetic protocols aim at incorporating additional light absorbing moieties already during the synthesis of PCN, for example, by introducing sulfur species , aromatic carbon moieties, creating copolymeric precursors with other N-containing molecules, or by varying the condensation conditions, especially the temperature or reaction medium composition. Secondly, post-synthetic approaches comprise chemical surface modification of conventional PCN, for instance, by hydrogenation or oxidation, sensitization with light harvesting compounds such as organic dyes , carbon dots or fullerenes, or various forms of post-synthetic thermal treatment. Although the condensation of PCN precursors at high temperatures (above 600 C) and the post-synthetic annealing of PCNs under similar conditions might seem very similar, the essential difference lies in the lower ammonia gas evolution from the material in the latter case. While the two approaches have somewhat similar effects on the material's optical properties if carried out under inert or reductive (NH3, H2) conditions or under vacuum, the reports on the performance of the resulting photocatalysts in terms of activity or visible-light response are contradictory and show little consensus regarding the optimal thermal treatment conditions. For example, Wang et al. reported a continuous increase of the photocatalytic H2 evolution rate with increasing temperature of PCN condensation up to 700 C. In accord with this result, P. Niu et al. demonstrated an activity enhancement of PCN post-treated at 750 C. Nevertheless, other authors observed a maximum activity at intermediate heat-treatment (condensation or post-treatment) temperatures of 500 C, 550 C, and 580 C, 600 C or 680 C, followed by a significant drop in photocatalytic activity. These discrepancies in the reported photocatalytic performance of PCNs with extended visiblelight absorption are clearly due to the lack of our knowledge on the nature of the colour centers that are responsible for the activation beyond wavelengths of ~450 nm in various PCNs. A number of plausible suggestions have been made to explain the red shift of the optical absorption edge, including increased condensation degree due to a partial residual NH2-groups removal, improved stacking allowing normally forbidden n-* transitions, and creation of N- or C-vacancies. Nonetheless, given the disordered nature of PCN polymers, it is difficult to unambiguously prove the distinct type of colour centers in the material, so that they often remain rather hypothesised than proven. However, the determination of the moieties or structural features responsible for the red shift is crucial for developing an appropriate synthetic or post-synthetic technique for the preparation of efficient visible-light active PCNs.