This review aims to be a comprehensive, authoritative, critical, and accessible review of general interest to the chemistry community; because the electrophilicity index is a very useful global reactivity descriptor defined within a conceptual density functional theory framework. Our group has also introduced electrophilicity based new global and local reactivity descriptors and also new associated electronic structure principles, which are important indicators of structure, stability, bonding, reactivity, interactions, and dynamics in a wide variety of physico‐chemical systems and processes. This index along with its local counterpart augmented by the associated electronic structure principles could properly explain molecular vibrations, internal rotations and various types of chemical reactions. The concept of the electrophilicity index has been extended to dynamical processes, excited states, confined environment, spin‐dependent and temperature‐dependent situations, biological activity, site selectivity, aromaticity, charge removal and acceptance, presence of external perturbation through solvents, external electric and magnetic fields, and so forth. Although electrophilicity and its local variant can adequately interpret the behavior of a wide variety of systems and different physico‐chemical processes involving them, their predictive potential remains to be explored. An exhaustive review on all these aspects will set the tone of the future research in that direction. In the domain of conceptual density functional theory, electrophilicity index is an important descriptor in describing the reactivity of a molecule as a whole. Its local variant, however, provides information about the reactivity of particular atomic sites, thereby analyzing the electrophile‐nucleophile reactions better. Their variation is explained in dynamical processes, excited states, confined environment, spin‐dependent and temperature‐dependent situations, biological activity, site‐selectivity, aromaticity, charge removal and acceptance, presence of external perturbation through solvents, external fields, and so forth.