A general description of nucleophilic reactions is developed using bond bundles, an extension of the quantum theory of atoms in molecules, allowing novel activating groups to be predicted for aromatic rings. Reactivity is found to be related to both the shape of the bond bundle between the substrate and leaving group and the presence of nonbonding regions. Closed bond bundles are shown to be more reactive than open ones, while nonbonding regions also increase reactivity. The advantage of this approach is that it can be employed to investigate all molecular and solid-state systems. By way of example I use this model to rationalize two anomalously reactive systems: strained heterocyclic rings and sulfide-activated aromatic rings.