Stacking of COF-5 layers into a 3D structure with different arrangements (Figure ) changes electronic properties of the materials. While the eclipsed stacking keeps almost unchanged inplane signatures of the kagome lattice, interlayer interactions induce strong band dispersion in the direction perpendicular to the layers. The band dispersion between  and A points in the Brillouin zone reaches almost 1 eV. As discussed above, this stacking is, however, energetically unfavorable. The two most stable high-symmetry structures of COF-5, inclined and serrated, strongly alter the electronic structure of the monolayer. In both cases, the in-plane kagome characteristic is no longer present along the -K-M- path. For serrated case, some kagome signature is visible for A-L-H-A path, however, these bands are not forming the band edges, as in the monolayer or eclipsed form. Out-of-plane dispersion is also present, but smaller than in the eclipsed stacking. For the serrated case, it is less than 0.5 eV. Band gap values and characters also change compared with the monolayer case: for the eclipsed structure, we calculated an indirect band gap of about 1.3 eV between  and A points, for the inclined structure, we obtained an indirect band gap of about 2.2 eV between X and  points, while serrated stacking gives also an indirect band gap of about 2.2 eV between M and  points. Band dispersion is of course desirable for nanoelectronic applications, because curved bands at the band edges result in light charge carriers (holes and electrons). While COF-5 single layer offers light electrons and heavy holes, which is interesting for production of short transport channel devices, the stacking introduces also light holes, which could be interesting for standard transistor applications. This survey on COF-5 shows that, just as in graphene, the proximity effect has a strong influence on the electronic structure of molecular framework materials. After the reports of COF-1 and COF-5, many routes to form 2D COFs have been developed. While a comprehensive summary of the literature would not fit into the scope of this article, we highlight here the most important steps that are crucial for the development of van-der-Waals science in 2D COFs. First, a large variety of coupling reactions has been developed in the field of 2D polymers (the term 2D polymers is commonly used for single layers of 2D COFs). We show a selection in Figure . Second, the large diversity of aromatic structures offers, besides honeycomb and square, also many other structural forms, including hexagonal and kagome. For detailed reviews, see Refs. [74-77]. Moreover, even the symmetry of the pores can be controlled by proper choice of linkers. This illustrates that there is no doubt that the potential structures offered by 2D COFs significantly exceed those of dense layered materials that are available in nature or have been synthesized. Third, in order to enable ballistic electron transport also inside the individual COF layers, it is important to remove the electron scattering centers, which typically are the atoms at the coupling sites. If they are perturbing the conjugation in the COF, electrons cannot flow and the electronic system is not much different to that of a molecular crystal. As shown in the groups of Jiang and Feng, fully conjugated 2D COFs are possible, e.g., if aromatic building blocks are coupled by using the Knoevenagel reaction. These works demonstrate that 2D COFs with fully conjugated electronic π-system can be realized in experiments, which means that it is possible to construct high-mobility single layer 2D COF semiconductors. It remains to be explored which consequences the proximity effect has, if these 2D COFs are stacked upon one another, both in homogeneous and heterogeneous stacks.