Like metal-organic frameworks (MOFs), covalent organic frameworks (COFs) are attractive materials for energy conversion and storage because they are widely tunable at the molecular level while being robust and structurally precise. Among this rapidly growing class of organic materials, 2D COFs have given momentum to the development of “all-single-site” heterogenous photocatalysts owing to their tunable optoelectronic properties, combined with ordered porosity. However, the “COF trilemma” – the challenge of designing COFs that are crystalline, stable, and functional – poses a major bottleneck to the rational design of new COF photocatalysts. In addition, since catalysis occurs at the local level (the “active site”) and is often driven by defects, insight into the local structure and control of disorder and defects is vital. This talk showcases our recent progress towards the design of COF photocatalysts for the hydrogen evolution reaction with a particular focus on structure–property–activity relationships. We will discuss structural (local and long range), optoelectronic and catalytic boundary conditions guiding our design of all-single-site COF photocatalysts and highlight the importance of real structure effects and defects in COFs and MOFs as a future design principle to create precisely tailored porous frameworks for catalysis and beyond.