Apart from Axiomatic Quantum Field Theory which is no longer as important as it was in the 1970s, Quantum Field Theory (QFT) is a general term for theories that describe interactions between particles. In many cases fundamentals of a QFT can be traced back to a combination of methods of classical Field Theory and/or Quantum Mechanics. Mostly Relativity is considered, too. The most outstanding feature of QFT is the so-called second quantization. While quantization in normal Quantum Mechanics is a computational procedure which describes the process of quantization of energy and momentum of particles, second quantization in QFT additionally describes the interaction between particles in terms of virtual quanta. This is done for instance with the help of creation- and annihilation-operators, Feynman graphs, path integrals, renormalization etc. QFTs which describe reality are gauge theories. The perfect example of a QFT is Quantum Electrodynamics which describes among others the interaction between charged particles by the interchange of virtual photons. The force being responsible for phenomena within Quantum Electrodynamics is the electromagnetic force. Together with characterizations belonging to the weak force and the strong force QFTs can be formulated which describe all known particles and phenomena under the roof of a so-called standard model. The ultimate goal of today’s physics is to unify elementary particle physics and gravitation which is ruled by another force, the gravitational one. The QFT belonging to this is sometimes called the grand unified theory and it is not completed yet.