Molecules with concealed antiaromaticity feature a formal 4n π-electron system (Figure , bold bonds), but additional structural units and/or conformational changes prevent this 4n π-electron system from turning the molecules antiaromatic. In other words, the additional structural units and the conformational changes conceal the antiaromaticity, sometimes even resulting in molecules that may also be described as locally aromatic or nonaromatic. Based on molecules described in the literature, three common structural motifs to conceal the antiaromaticity can be identified: 1. Locally aromatic units sharing 2 π-electrons with the 4n π-electron system: Numerous molecules that conceal their antiaromaticity in this way have been reported, ranging from comparatively small molecules like dibenzocyclooctatetraene and tetraoxa [8]circulene to large molecular propellers (Figure ). Formulated as a molecular design strategy starting from 4n annulenes, this can be described as ortho-fusing aromatic units with the 4n π-electron system. 2. Locally aromatic units sharing 4 π-electrons with the 4n π-electron system: Conjugated macrocycles often conceal their antiaromaticity in this way, including paracyclophanetetraene and cycloparaphenylenes (Figure ). Formulated as a molecular design strategy, this can be described as creating locally aromatic units along the 4n π-electron system by introducing 2π-electron bridges. In both cases, fragments of the 4n π-electron system are integrated into locally aromatic subunits with [4n+2] π-electrons (Figure , grey shadings), but the integration of 4 π-electrons is considered to have a more pronounced effect on the antiaromaticity than the integration of 2 π-electrons. Both structural motifs can also be present in the same molecule. Aromatic units may also be fused with more than one 4n π-electron system. In addition to these units, the molecules are often able to further conceal the antiaromaticity by conformational changes, unless structural restrictions prevent the formal 4n π-electron system from adopting a non-planar conformation.