This section is devoted to the most important aspects concerning the combination of SERS substrates and MOFs materials. Specific and more detail reviews individually addressing these topics can be found in the literature, and they are suggested to the reader. Light couples with molecules via their polarizability leading to either elastic or inelastic scattering. One of the possible inelastic scattering routes is Raman scattering. It is a weak process that is roughly 10 6 times less probable than elastic Rayleigh scattering and occurs at narrow wavelength bands that are shifted by discrete energy levels from that of the impinging light. These shifts are characteristic for chemical structure of the molecules and thus spectral analysis of Raman-scattered light can be used as a powerful identification tool (see Figure ). Raman scattering is commonly described based on the interaction with vibrational states of the molecule, and it is associated with absorbing (or releasing) vibrational energy in Stokes (or anti-Stokes) series of bands. If the wavelength of impinging light matches the energy of the electronic absorption of the molecule, resonant Raman scattering can occur with greatly increased efficiency. An alternative means for (arguably) more versatile enhancement of inherently weak Raman scattering is pursued by using metallic nanostructures. They exhibit collective oscillations of charge density coupled with electromagnetic field that are referred to as surface plasmons. Their resonant optical excitation allows for tight confinement of electromagnetic field and respective strong enhancement of its intensity. Plasmonic metallic nanostructures can be formed on continuous metallic films supporting propagating surface plasmons (PSPs) or by metallic nanoparticles with sharp edges, tips, or crevices generating the so-called localized surface plasmons (LSPs). When a molecule is placed within the intense surface plasmon field, the yield in Raman scattering is greatly enhanced as it scales with the electromagnetic field amplitude to the power of four. This effect is the basis for Electromagnetic enhancement Mechanism (EM) and it was reported to allow for extraordinarily high enhancement factors of 10 6 -10 12 . The rich diversity of plasmonic structures that can be obtained either by chemical synthesis of colloids, by their self-assembly to ordered structures or by nanofabrication techniques, allows for the precise engineering and control of wavelength bands where the plasmonic field intensity enhancement occurs. It also allows for the design of specific locations where the electromagnetic field enhancement peaks that are referred to as plasmonic 'hotspots' (see Figure ). Besides the EM contribution to the enhancement mechanism behind SERS, the Chemical enhancement Mechanism (CM) also affects Raman scattering efficiency. It relates to the formation of charge-transfer complexes between the target molecule and the metallic surface. As shown in Figure , there is a whole set of energy transitions possible, either from the metal to the molecule or vice versa (including intrinsic transitions within the molecule related to already established resonant Raman scattering). SERS charge-transfer in metal-molecule complexes is maximized when the metal state involved is at the Fermi energy. Ground-state charge-transfer (µGSCT) is also possible when the energy levels of the SERS substrate and the analyte are aligned. When the energy of the excitation wavelength matches the energy difference for a charge transfer (CT) between the metal and the molecule in any direction, photoinduced charge transfer (µPICT) processes take place. This µPICT process is of significant importance, and we will come back to it later. CM is also possible with semiconductor-based SERS substrates. In addition to the molecular resonance transitions (µmol), µGSCT, and µPICT, exciton resonances (µex) are also possible (excitons are holeelectron pairs generated in solids by optical excitation). Orchestrating these transitions is one of the most critical aspects for SERS-substrates design. It is important to highlight here that the classification of the enhancement mechanisms in terms of EM or CM is a simplification and these processes are related to each other. SERS should be regarded as a concomitance of all these effects, affecting in a greater or lesser extent the total enhancement according to the wavelength employed and the molecule-surface interaction.