Introduces the theoretical concepts, based on quantum mechanics and the special theory of relativity, needed to describe relativistic particles and their interactions. The course starts out with the Klein-Gordon and Dirac field equations, describing free scalar particles and fermions respectively, and their quantization. It is then shown how interactions can be included in perturbation theory and how they can be described through Feynman diagrams. These techniques are then applied mainly to calculate tree-level processes in quantum electrodynamics. The course ends with a short introduction to higher order processes and radiative corrections.