Monocyclic molecules with a delocalized π-electron system of [4n+2] π-electrons, where n is a nonnegative integer, are aromatic according to Hückel's rule, whereas with 4n π-electrons they are antiaromatic. These rules of ground-state aromaticity are reversed in the lowest singlet and triplet excited states, where molecules with a cyclic delocalized system of 4n π-electrons are expected to be aromatic and with [4n+2] π-electrons to be antiaromatic according to what is known as Baird's rule. In all cases, the π-electron system has to be planar or close to planar for the π-electron delocalization to occur. While the π-electron delocalization results in energetic stabilization in the case of aromatic molecules (compared to an acyclic structural analogue), it leads to destabilization in the case of antiaromatic molecules. The lower stability of antiaromatic molecules can be rationalized by their two degenerate non-bonding singly occupied molecular orbitals (SOMOs) that are higher in energy than the highest occupied molecular orbital (HOMO) of the corresponding localized structure. In contrast, aromatic molecules feature a closed-shell structure with two degenerate bonding HOMOs that are lower in energy. As a consequence, antiaromatic molecules try to escape antiaromaticity in different ways, often by adopting a more stable non-planar conformation with localized π-electrons (and a closed-shell structure) or by undergoing reactions. For example, cyclooctatetraene adopts a boat-like non-planar conformation to escape antiaromaticity (Figure ), whereas cyclobutadiene is known to dimerize (Figure ). 13 Besides monocyclic antiaromatic molecules with two degenerate SOMOs and a diradical character y0 of 1 (referred to as truly antiaromatic molecules in this article), there are molecules with a formal 4n π-electron system but reduced diradical character (between 0 and 1). These molecules -including pentalene, dibenzopentalene, s-indacene, and biphenylene (Figure ) and their derivatives as well as different cyclooctatetraene derivatives -are often also referred to as antiaromatic, even when the diradical character is low. In contrast to truly antiaromatic molecules, which may only be isolated under cryogenic conditions, these polycyclic molecules are more stable and isolated more readily. However, the classification of such molecules as antiaromatic has been criticized, particularly for polycyclic molecules that feature both formal 4n and [4n+2] π-electron systems (such as dibenzopentalene and biphenylene, Figure right). At the centre of the controversy is the question of whether any molecule with a formal 4n π-electron system that exhibits some degree of paratropic current (e.g. in ACID plots or according to slightly shifted NMR signals) can be classified as antiaromatic -a viewpoint often held by experimental organic chemists -or whether the term should be reserved for molecules with a 4n π-electron system that fulfil not only the magnetic criterion but also the structural, energetic, and electronic criteria of (anti)aromaticity. While the latter view follows a rather strict holistic approach and is in better agreement with the definition of (anti)aromaticity, the former is less restrictive and often based on the observation that the molecules in question behave like antiaromatic molecules under certain conditions, which explains why this viewpoint is often held by experimental chemists. In an approach that generalized Hückel's rule to polycyclic molecules, such molecules were previously classified as intermediate, showing partially aromatic character, but this classification does not provide information about the properties and behaviour of the molecules. Other terms that have been suggested in informal discussions, including attenuated antiaromaticity or pseudo-antiaromaticity, are limited in scope and not suitable for the development of a more general framework.