Aromatic π-stacking is a key supramolecular motif, yet the fundamental intermolecular forces at play in ππ interactions continue to be debated. A recurring question is whether π-π interactions constitute a unique intermolecular force that is distinct from "ordinary" dispersion. Quantum-mechanical calculations suggest that π-stacking is driven by dispersion but enhanced by the planar geometries of aromatic moieties, in what has been called the "pizza-π" model for stacking. This is consistent with the existence of stacking interactions between molecules that are planar but not aromatic. A persistent feature of π-π interactions is "slipstacking", where two arene moieties adopt a paralleldisplaced geometry as illustrated for the benzene dimer in Fig. . In that system, the energetic preference for offset-stacking is smaller than 1 kcal/mol but it is considerably larger in polycyclic aromatic hydrocarbons (PAHs). The parallel-displaced motif is observed repeatedly in other π-π interactions: between aromatic side chains in protein crystal structures; in transition metal complexes with aromatic ligands; in dimers of pyridinoids, 31 macrocycles, and circumcoronenes; for aromatic and anti-aromatic π systems atop graphene; and in layered materials such as covalent organic frameworks. Understanding the intermolecular forces that drive offset π-stacking is crucial for achieving rational control of these and other supramolecular structures. An explanation for why the parallel-displaced geometry of benzene dimer is preferred over the cofacial configuration was put forward long ago by Hunter and Sanders, based on a simple potential energy model in which London dispersion competes with quadrupolar electrostatics. Dispersion is most attractive in the faceto-face geometry (Fig. ) but quadrupole-quadrupole interactions are repulsive in that configuration. The Hunter-Sanders model posits that displacement along a cofacial sliding coordinate reduces quadrupolar repulsion without sacrificing too much of the favorable dispersion interaction, which decays rapidly (as R ) with interatomic distance. This idea was quickly assimilated into the lore of organic chemistry despite limited experimental evidence, mostly consisting of correlations between relative stacking energies (∆∆G • , measured in chemical double-mutant experiments ) and Hammett parameters for the substituents. Occassionally, electrostatic potential calculations (as in Fig. ) have been used to justify this explanation, although that approach has been criticized as ambiguous. Other experimental results have been more ambiguous or contradictory. In addition, the model potential introduced by Hunter and Sanders fails to reproduce the offset-stacking behavior that is observed in PAHs ranging from naphthalene dimer to benzene atop circumcircumcoronene, (C 6 H 6 )• • • (C 96 H 24 ). It also fails to explain why offset-stacking is preferred in the (C 6 H 6 )• • • (C 6 F 6 ) heteromolecular dimer. In that system, the polarity of the C-F bonds reverses the sign of the quadrupolar electrostatic interaction, making it attractive rather than repulsive in the cofacial geometry. A consistent explanation for these observations, and for offset-stacking in the porphyrin and [18]annulene dimers that were also considered by Hunter and Sanders, is that slip-stacking behavior is not under electrostatic control at all. In this "van der Waals" (vdW) picture, offset-stacking behavior is driven by a competition between dispersion and Pauli repulsion, both of which are short-range forces. This provides a more satisfying explanation for the persistence of slip-stacking in protein crystal structures, because the short-range forces are ever-present despite differences in long-range electrostatics engendered by a protein environment. This prediction was recently confirmed in a broad computational survey of π-π interactions in proteins. Other aspects of the Hunter-Sanders picture also appear to be inconsistent with detailed quantummechanical calculations. For example, putative electrostatic control of π-π interactions suggests that stacking energies should be modulated by electron donating or withdrawing properties of substituents, as these impact the π-electron density of the arene. However, Wheeler co-workers have shown that substituent effects on stacking energies are better understood using a "direct interaction model", in which through-space interactions between the substituent and the opposite arene are more important than a substituent's effect on the π-density of its own aromatic ring. This idea has antecedents in the through-space "polar-π" model of Cozzi and Siegel, the evidence for which consists mostly of correlations between ∆∆G • and Hammett parameters for the substituents, i.e., the same evidence used to support the Hunter-Sanders model. While it was argued that the polar-π model is consistent with "chemical intuition" regarding electron donating or withdrawing effects, Wheeler and Houk have shown how this intuition can be subverted by the spatial orientation of the substituent moiety. The Wheeler-Houk direct-interaction model has garnered support from experiment and has been confirmed at various levels of ab initio theory. It is more nuanced than the picture afforded by Hammett parameters, which do exhibit rough correlations with calculated stacking energies although there is some suggestion that these correlations may be fortuitous. Calculations reveal that molecular polarizability plays a significant role in stacking interactions, due to the importance of dispersion, but some have argued that its importance is diminished by solvation effects. This should be contrasted with the T-shaped benzene dimer (Fig. ), where substituentinduced changes in intermolecular interaction energies do appear to be under electrostatic control and correlate well with Hammett parameters alone. In more complicated systems, these parameters may be insufficient to explain substituent effects on experimental stacking energies. Despite this progress toward understanding π-π interactions in detail, the Hunter-Sanders paradigm remains textbook material that is invoked widely and unapologetically, even in recent literature. The model benefits from its "just-so" simplicity, yet ultimately rests on a fallacy. At vdW contact distances, including typical π-stacking distances of ∼ 3.4 Å, charge densities for the two arene moieties interpenetrate, invalidating the multipolar approximation. As such, electrostatic interactions cannot be reliably (or even qualitatively) described by leading-order molecular multipole moments. This can create situations in which the quadrupole-quadrupole interaction differs in sign as compared to the exact electrostatic interaction computed using monomer charge densities. This is precisely the situation for benzene dimer and small PAHs at 3.4 Å separation, where the genuine electrostatic interaction is attractive in the cofacial arrangement, in direct contradiction to the Hunter-Sanders prediction. The polar-π picture also invokes the arene's quadrupole moment and is similarly invalid for short-range interactions. An electrostatics-driven picture may take hold for larger arene-arene separation, 144,147 but this is not relevant at vdW contact distances.