Creating or connecting together large organic molecules, as polycyclic aromatic hydrocarbons (PAH), readily on surfaces arises as a step of paramount importance towards a true advance in the field of nanotechnology, and particularly of molecular electronics. On-surface synthesis can be regarded as an efficient means to build new molecular species by using bottom-up strategies. In particular, temperature-driven surface-catalysed cyclodehydrogenation (CDH) processes have burgeoned in last years as a novel and powerful route to efficiently transform (hetero-)aromatic molecular precursors into a large variety of a-la-carte hierarchical nanostructures: from fullerenes and triazafullerenes, aromatic domes, or nanotubes, to polymeric nanonetworks and, depending on the specific precursor utilized, even to pristine and functionalized graphene. In the first Section of this Chapter, the foundations and main aspects of the on-surface synthesis methodology and CHD reactions are revised, as well as the current status in the field up to the date. In Section 2, most advanced first-principles theoretical tools currently available for the characterization of cyclodehydrogenation processes will be summarized and described, including the novel theoretical strategies to monitorize cyclodehydrogenation reaction paths and to calculate STM images. The following two Sections will report on a very recently paradigmatic example related to the tailored formation of N-doped nanoarchitectures by diffusion-controlled on-surface (cyclo-)dehydrogenation of heteroaromatics, where the strength of the PAH-substrate interaction dramatically rules the competitive reaction pathways (cyclodehydrogenation versus dehydrogenative polymerization). On the basis of those findings, it will be fully described the stepwise formation of N-doped nanohelicenes, nanographenes, nanodomes, molecular networks and graphenes from the same heteroaromatic precursor through subsequent dehydrogenations on Pt(111) upon thermal annealing. The combined experimental (in-situ UHV-STM, XPS and NEXAFS) and detailed computational DFT-based studies provide a full atomistic and chemical description of the intermediate reaction stages along the dehydrogenation path.