In textbooks, nucleophilic aromatic substitution (SNAr) reactions are generally explained to proceed through a two-step additionelimination sequence via a discrete, non-aromatic Meisenheimer complex. This intermediate is usually stabilized by electronwithdrawing groups (EWGs), leading to a minimum along the reaction coordinate. Recently, however, an increasing number of concerted nucleophilic aromatic substitutions have been reported, featuring better leaving groups and less stabilizing EWGs. These findings are supported by, for example, a study involving qualitative Marcus theory (Figure ), which suggests that the reaction pathway depends critically on the energy of the Meisenheimer complex relative to the "potential energy surfaces" (in the language of the valence bond theory in modeling reactivity) of the starting materials and products. When this complex is lower in energy, the reaction proceeds stepwise; if it is higher, the mechanism is concerted, with the transition state becoming the minimum of the Meisenheimer curve. Borderline cases, such as when a stabilized anion is adjacent to a good leaving group, show that the Meisenheimer complex may merely form a shallow minimum or shoulder on the reaction coordinate, indicative of the blurred lines between mechanisms. In reactions consistent with concerted SNAr, the transition state structures were found to possess characteristics of a Meisenheimer intermediate rather than those of the aromatic starting material. When the Meisenheimer structure is less stable, reactions tend to be concerted. If this structure is highly stabilized but leaving group elimination is facile, a borderline situation arises, underscoring that the distinction between concerted and stepwise mechanisms is not dichotomous but depends on the relative energy levels between the reactants, products, and Meisenheimer ionic states. In some instances, the intermediate could behave more like a transition state. This continuum is further explored in another study that surveyed various substituents and nucleophiles, mapping out the range between concerted and stepwise mechanisms. A concerted mechanism can be synchronously or asynchronously concerted, with varying degrees of asynchrony; when it becomes highly asynchronous, it transitions to a stepwise mechanism. With the idea of a blurred dichotomy, many mechanistic exceptions should no longer be surprising.