Quantum field theory describes the physics of all elementary particles in nature. This course provides an introduction to the most important features of quantum fields. We start by generalizing non-relativistic quantum mechanics (as covered in the Theoretical Physics II and Quantum Mechanics II lectures) to the relativistic case and discussing the relevant properties of the Lorentz group. This is followed by the quantization of scalar and fermion fields, using both the canonical formalism and the path integral. We conclude with the discussion of gauge fields and derive the Feynman rules for quantum electrodynamics, using which we calculate the anomalous magnetic moment of the muon -- one of the most important quantities in current high energy particle physics research.