Hydrogen is an unavoidable impurity in all semiconductors. It interacts with intrinsic defects (from monovacancies to dislocations), with impurities (shallow dopants, deep centers, even electrically inactive impurities), with the crystal, and with other H interstitials. These interactions profoundly affect the electrical and optical properties of the host. Conventional experimental techniques used to study the properties of hydrogen (EPR, IR or Raman spectroscopy, etc.) have provided information on a number of H-related defects. Theory has played a major role in these studies, not only by confirming the models proposed on the basis of experimental data, but often by explaining the data altogether or predicting new features. So far, μSR has provided fundamental information on isolated hydrogen-like species in many semiconductors. It would be wonderful if the spectroscopic signature of muonium-impurity pairs could be identified or if quantitative information regarding the stability of the various charge states of muonium could be obtained.