Concealing the antiaromaticity increases the stability of the molecules during synthesis, purification, and processing while retaining desirable properties of antiaromatic molecules (e.g. excellent stability in the doubly charged states), but which properties are retained depends on how strongly the antiaromaticity is concealed. The integration of fragments of the 4n π-electron system into locally aromatic subunits is considered to lead to a tug-of-war between the local aromaticity of the subunits and the global antiaromaticity of the 4n π-electron system, which can be influenced by (i) the choice of subunits and the strength of their local aromaticity, (ii) the number of aromatic subunits in relation to the number of π-electrons in the 4n π-electron system, and (iii) the ability or disability of the 4n π-electron system to undergo conformational changes. Depending on these factors, different degrees of concealment can be achieved. The antiaromaticity can be strongly concealed like in cycloparaphenylenes (Figure centre), which feature a high number of strongly aromatic subunits in relation to the number of π-electrons in the formal 4n π-electron system, or very weakly concealed like in Ni(II) norcorroles (Figure right), which cannot undergo conformational changes due to structural restrictions and only features two locally aromatic subunits in the formal 16 πelectron system, resulting in a relatively high diradical character y0 of 0.127. If the antiaromaticity is strongly concealed, the molecules may also be seen as locally aromatic (or nonaromatic in cases like cyclooctatetraene), but also for these molecules the term concealed antiaromaticity can provide useful guidance to understand the experimental properties, as these molecules can still behave like antiaromatic molecules under certain conditions. The degree of concealment determines the conditions under which the molecules can behave like antiaromatic molecules, so the conditions under which they can reveal their concealed antiaromaticity. Based on these conditions, three types of concealed antiaromaticity are distinguished here: 1. Type I-CA: Concealed antiaromaticity revealable in redox reactions 2. Type II-CA: Concealed antiaromaticity revealable upon photoexcitation 3. Type III-CA: Concealed antiaromaticity revealable in intermolecular interactions The different types of concealed antiaromaticity are not mutually exclusive. Molecules with Type I-CA often also feature Type II-CA and vice versa. Molecules with Type III-CA will normally also feature Type I-CA and Type II-CA. The three types of concealed antiaromaticity are discussed in more detail below, along with a selection of examples identified in the literature (see also Figure ). Baird aromatic upon photoexcitation. Type III-CA: [24]Annulene tetroxide and 5,6,17,18bisdehydrotetrathia [24]annulene planarize and show chain-like/dimer stacking in the solid state.