Quantum field theory is an extension of quantum mechanics from point particles to fields, such as the electromagnetic field. Development of a new theory was needed because ordinary quantum mechanics cannot describe creation or annihilation of elementary particles and is incompatible with Einstein's theory of special relativity.
Quantum field theory has been developed during 1935-1955 by several physicists, including Dirac, Pauli, Dyson, Schwinger, and Feynman. It is the foundation of modern theories of elementary particles affected by electromagnetism, weak and strong interactions. The quantum field theory description of electromagnetic phenomena, or the theory of quantum electrodynamics, is one of the best tested theories.