The electron, as we have seen, is one of the six leptons of the Standard Model, and using the Schrödinger equation and thus all the tools of quantum mechanics, it is always possible, albeit in probabilistic terms, to define its properties always keeping in mind its dual wave-particle nature. However, the electron remains something elusive and incomprehensible; after all, the question from which we started, i.e., what is an electron, still has no definitive answer. The ultimate ambition of physics is to arrive at a theory that describes everything, if that were even possible. The theory of everything, the one that would integrate gravity, i.e., general relativity and quantum mechanics, is not yet there, but Quantum Field Theory (QFT) is a sophisticated and fascinating theoretical model that accounts for the nature of fundamental particles. The QFT can be applied in different cases; the theory that explains electromagnetism is called quantum electrodynamics, (QED), while the theory that explains the strong force is the quantum chromodynamics, (QCD).