It is a pleasure to announce the Special Issue “Quantum Field Theory XXI”. Since its first applications in the XX century, and the crowning with the formulation of the Standard Model of electroweak and strong interactions, quantum field theory has evolved, both in meaning and intention. Its perturbative and nonperturbative incarnations have been playing an undisputed role in modern theoretical physics, not only in the refinement of the Standard Model and of its supersymmetric extensions, but also in the quest for a quantum theory of gravitation, carried on in the programs of string theory, supergravity, and many other quantum gravities (nonlocal quantum gravity, asymptotic safety, group field theory, and so on), and in our understanding of astrophysical and cosmological processes, ranging from the Big Bang to stellar evolution, and from the cosmological constant to gravitational waves.
This Special Issue aims to recapitulate part of these achievements and offer a perspective on near- and far-future applications of quantum field theory in the present century. Review and perspective papers are especially welcome.