Electron delocalization is a fundamental concept in chemistry. It is deeply rooted in many important chemical phenomena e.g. aromaticity, conjugation and the reactivity of a vast number of inorganic, organometallic and organic compounds. This review focuses on those theoretical studies in which electron delocalization indicators provide valuable insigths in the understanding of electrophilic aromatic substitutions. The considered approaches to deal with electron delocalization include primarily the quantum theory of atoms in molecules but also briefly the analysis of the electron localization function, the topography of the molecular electrostatic potential, the anisotropy of the induced current density and GIAO-derived charge delocalization modes. We go over the characterization of the electron delocalization in the most important intermediates of the electrophilic aromatic substitutions namely the Wheland intermediates and the complexes formed by aromatic molecules and electrophiles. Then we discuss the relation between aromaticity and electrophilic aromatic substitutions. Finally, we give an account of the importance of the electron delocalization in the effect of the substituents on the regiochemistry and reactivity of an aromatic molecule towards an electrophilic attack. Besides reviewing the literature in an extensive way, it is our hope that this review will suggest directions of further progress in the field.