Supercapacitance can be separated into two principal contributions: electric double-layer capacitance (EDLC) and pseudo-capacitance (Fig. ). EDLC is due to the electrostatic charge separation and the formation of an electric double-layer at the electrode-electrolyte interface (Fig. ), while pseudo-capacitance involves rapid reversible Faradaic reactions occurring at the electrode surface (Fig. ). Pseudocapacitive mechanisms with redox pseudo-capacitance, as occurs in (c) RuO2, and (d) MOFs with redox-active moieties. EDLC primarily arises from charge separation occurring at the electrode-electrolyte interface, where ions in the electrolyte are adsorbed onto the electrode surface, forming an electrical double layer. This electrostatic charge separation leads to electric double-layer capacitance, which is typically associated with moderate capacitance values (of order 100 F/g) and fast charge/discharge rates (seconds to a few minutes). The electrode charge is in the form of delocalized electrons or holes as found for porous carbon electrodes and it is compensated by the electrolyte ions via counterion insertion, co-ion removal or ion-exchange mechanisms. While EDLC has also been reported for MOF electrodes, these materials have a more inhomogeneous charge density distribution with more complicated polarisation phenomena. On the other hand, pseudo-capacitance involves rapid reversible redox reactions of electroactive species at the electrode surface, which can store additional charge beyond the electrical double layer. Pseudo-capacitance is associated with Faradaic processes and typically results in higher capacitances (much larger than 100 F/g) than pure EDLC charge storage, but slower charge/discharge rates due to the involvement of redox reactions. This mechanism is active in metal oxides such as NiO, MnO2, and RuO2, where the electrode charge is in the form of localised small polarons. Interestingly, similar building blocks can be incorporated in MOFs, and it has been reported that MOF-based supercapacitors can exhibit pseudo-capacitance. Assigning the active charging mechanism(s) in supercapacitors can require a combination of techniques. Initial experimental evidence for pseudo-capacitance can include redox peaks in cyclic voltammetry curves, non-linearities in Galvanostatic charge/discharge measurements, and low-frequency signatures in electrochemical impedance spectra. In materials modelling, the pseudocapacitive contribution can be realised by direct first-principles simulations, e.g. density functional theory (DFT), whether there exist oxidation state changes in the electrode or charge transfer between the electrode and electrolytes takes place.