  Muonium, the atom which consists of a positive muon and an electron, has been
discovered by a team led by Vernon W. Hughes in 1960. It is in many respects
the most ideal atom available from nature. Due to the close confinement in the
bound state muonium can be used as an ideal probe of electro-weak interaction,
including particularly Quantum Electrodynamics, and to search for additional
yet unknown interactions acting on leptons. Recently completed experiments
cover the ground state hyperfine structure, the 1s-2s interval and a search for
spontaneous conversion of muonium to antimuonium. The experiments yield precise
values for the fine structure constant, the muon mass and its magnetic moment.
The results from these precision measurements have provided restrictions for a
number of theories beyond the Standard Model of particle physics. Future
precision experiments will require new and intense sources of muons.
