Considering the global need to move from fossil fuels to renewable sources of energy, fuel cell chemistry and in particular the oxygen reduction reaction (ORR) are highly active fields of research. Iron and nitrogen doped carbon-based (FeNC) catalysts show high ORR activity and have thus emerged as a promising alternative to platinum-based catalysts in fuel cells. Due to a pyrolysis step in their preparation, FeNC catalysts are amorphous materials with different iron phases, which hampers the structural and electronic decipherment of the active site. Previous experimental and theoretical studies have deduced that the ORR is driven by FeN 4 sites, where a single iron ion is coordinated via four nitrogen donors and embedded into a carbon matrix. However, their exact nature in terms of geometry beyond the FeN 4 -coordination and electronic structure remains unclear. For instance, FeN 4 centres where N is bound in five-membered rings (pyrrolic, often denoted as FeN 4 C 12 ) or in six-membered rings (pyridinic, often denoted as FeN 4 C 10 ) are discussed. Many quantum chemical studies have developed models that seek to describe active sites and their reaction mechanisms. Recent quantum chemical work coupled to in situ and operando Mössbauer spectroscopy studies on different FeNC preparations has shown that pyrrolic FeNC sites form a thermodynamically and spectro-scopically consistent active site, in agreement with more recent works. Mössbauer spectroscopy provides insights into the iron coordination environment, specifically the oxidation state, spin state and more subtle details concerning the asymmetry of the electron density distribution. In studies of FeNC catalysts, Mössbauer spectroscopy is used to elucidate different iron phases in FeNC catalysts as well as the changes of characteristic signatures under in situ and operando conditions. Synthetically, FeN 4 sites are easily accessible in small molecular complexes such as porphyrin, phthalocyanine or phenantroline ligand spheres, which lack the extended π system expected to be present in the carbon matrix of FeNC materials. The interpretation of Mössbauer data on FeNC catalysts therefore relies on comparisons with such model complexes, or, more recently, on the comparison to predicted data from quantum chemical calculations where the extended π system can be included in the model. With a novel preparation strategy using active site imprinting and transmetalation, MNC catalysts with minimal inorganic side phases were produced. This results in materials with structurally better defined MN 4 sites, e.g. ZnNC systems with a preference for pyrrolic sites, which are subjected to transmetalation with iron and subsequent heat-activation. It is reported that the structure of the MN 4 environment is conserved to a significant degree and that the formation of elemental iron or iron carbide side phases is avoided. In the light of growing evidence for pyrrolic coordination, computationally developing suitable and representative active site models for such FeN 4 materials in general, however, remains challenging. Even for the same type of nitrogen donor, a wide array of different active site models can be constructed and evaluated computationally. Since the introduction of a five-membered ring necessitates the incorporation of an accompanying defect, e.g. a sevenmembered ring, to ensure that the remainder of the graphene plane remains structurally sensible, important variables in constructing active site models are the type and number of defects. Three representative active site models are shown in Figure . In models 1 and 2, the D 4h symmetry is kept in the first coordination sphere of the pyrrolic FeN 4 centre akin to a porphyrin complex, but with seven-membered rings at different positions. In model 3, the local symmetry of the FeN 4 centre is reduced to D 2h resembling a porphycene complex and the connectivity of the four nitrogen-containing rings is restructured to fit into the D 6h symmetric graphene sheet. Open questions to be addressed in this work are to what extent different active site models with pyrrolic active sites can be discerned by Mössbauer spectroscopy, and whether the Mössbauer parameters can be rationalized in terms of electronic structure characteristics.