In this review the authors have provided an authoritative overview of the field focused on studying the diffusion and dynamics of pi-conjugated molecules on surfaces. The review is exceptionally well written and structured, providing first a deep explanation on the fundamental aspects in molecular diffusion, the utilized theoretical models that are used to predict such complex processes, and the experimental techniques to characterize them. After that, the authors summarize the state-of-the-art for different types of pi-conjugated molecules from small compounds (such as benzene and five-membered rings) to larger and more complex systems such as phthalocynanines, porphyrins and poly-aromatic hydrocarbons of different structure. The authors exquisitely disentangle the different aspects that are involved in determining molecular diffusion and mobility, and how such different variables come into play by increasing molecular size and complexity and by moving between different types of substrates or from low to high temperature. As previously mentioned the review is authoritative, but also comprehensive and very accessible, which are both key aspects for a general audience platform such as Nanoscale Horizons.

Because of all this, I strongly recommend the acceptance for publication of this excellent review article. I only have some minor comments that the authors may consider to improve this already excellent report. These are the following:

1) Related to Table 1, it might be worth adding a small sign or colour to the specific molecular name (left column) associated to the molecular geometry displayed in the corresponding right column. This will facilitate the reader to associate which specific compounds are being displayed.
2) In section “Polycyclic aromatic hydrocarbons” the authors write “The TS correction yields ECC = 74.9 meV and ECH = 95.8 meV, revealing stronger binding at benzene-like sites due to their distinct local chemical environment. While this size-dependent enhancement implies stronger adsorption for larger PAHs as also confirmed by experimental observations,…”. If the binding energy is larger for those atoms that are bound to hydrogen atoms (ECH > ECC) – that is, for edge C atoms – the absorption energy *per atom* should be larger for the smaller molecules, as the ratio NH/NC will be larger for those. If the authors are referring to the total adsorption energy (instead of the adsorption energy per atom), that should be specified because, as it is written now, it seems they are referring to the “per atom” quantity, which is referenced right above these lines in the main text.
3) The activation energies shown in Table 5 and in other parts of the text are of the order of 1 eV. Now, this energy is well above kT (25 meV) at room temperature. So, with such activation energies, how is it possible that the molecules are still “able” to diffuse at room temperature or below? Maybe the explanation to such apparent contradiction could be included somewhere in the main text, because it might generate doubts for the general reader outside the field of molecular diffusion on surfaces.
4) Right before the Conclusion section, the authors discuss various fields where molecular diffusion of organic compounds plays a key role. I would suggest to also mention the field of bottom-up on-surface synthesis of carbon nanostructures, such as 2D conjugated polymers (Nature Materials 2020, 19, 874–880. https://doi.org/10.1038/s41563-020-0682-z) or graphene nanoribbons (Nature 2010, 466 (7305), 470–473. https://doi.org/10.1038/nature09211). In this area, the dynamics of the initial molecular building blocks used to built such nanostructures is absolutely key in the bottom-up synthesis process (Journal of Physical Chemistry Letters 2023, 14 (19), 4462–4470. https://doi.org/10.1021/acs.jpclett.3c00344). Therefore, I think it is worth highlighting this field as another area of materials science where the insights of molecular diffusion on surfaces could play a major role in the future.