ABSTRACT
The muon is a second generation fundamental fermion which belongs together with the electron and tauon to the group of charged leptons. The muon itself and the muonium atom, its bound state with an electron, have been widely used to investigate fundamental laws and symmetries in physics. Since all their known interactions can be calculated very accurately, they have also been employed to determine very accurate values of fundamental constants. The basic interactions between the fundamental fermions are mediated by gauge bosons the graviton, the photon, the W- and Z-bosons and eight gluons. Exotic interactions between electron and "nucleus" can contribute to the level energies. The deviations of the real atomic levels from the fine structure level energies as predicted in the Dirac theory and corrected for a finite nuclear mass can be calculated to high precision within the framework of quantum electrodynamics. The Lamb shift is largest for s-states, since they have a finite probability density at the origin.
