  We present an updated value of the Muonium 1S-2S transition frequency,
highlighting contributions from different QED corrections as well as the large
uncertainty in the Dirac contribution, stemming from the uncertainty of the
electron-to-muon mass ratio. Improving the measurement of this spectral line
would allow to extract a more accurate determination of fundamental constants,
such as the electron-to-muon mass ratio or, combined with the Muonium hyperfine
splitting, an independent value of the Rydberg constant. Furthermore we report
on the current status of the Mu-MASS experiment, which aims at measuring the
Muonium 1S-2S transition frequency at a 10 kHz uncertainty level.
