The preparation of 2D polymers has been explored at several interfaces, as confining polymerization to 2D can template or catalyze the bond formation. Central design considerations for on-surface 2D polymerizations include a rational design of the precursor as well as a suitable choice of the substrate and the activation method for the covalent coupling. Recent progress in surface and interface polymerization confirms the high feasibility of constructing 2D conjugated polymers from appropriate organic monomers for photocatalysis. The fundamental understanding of the reaction mechanisms and crystallization processes at the (sub)molecular level is essential for a predictive synthesis and for tailoring material properties for applications in photocatalysis and (opto)electronics.