In this chapter, Quantum Electrodynamics (QED) is discussed which is the best tested quantum field theory.

One of the most fundamental concepts in particle physics is the Gauge Principle which allows to introduce fundamental interactions by requiring invariance under (to be specified) local phase transformations. This is discussed in quite some detail for the case of QED.

Next, the complete form of the QED Lagrangian is discussed.

From the equation of motion for a Dirac field interacting with an external electromagnetic field the Feynman Rules are motivated.

The chapter closes with calculations of QED processes using the Feynman Rules.