  The electromagnetic interactions of electrons and muons can be described to
very high accuracy within the framework of standard theory, in particular
within the hydrogen-like muonium atom. Therefore precision measurements allow
to test basic interactions in physics and to search for yet unknown forces.
Accurate values for fundamental constants can be obtained. Results from
experiments on the ground state hyperfine structure and the 1s-2s intervals in
muonium are described together with their relations to a new measurement of the
muon magnetic anomaly.
