We investigated intersubband transitions in the valence band of Si1-xGex quantum wells. After a brief review of the recent achievements related to intersubband transitions, we present an original technique, photo-induced IR absorption spectroscopy, to study intersubband transitions in both doped and undoped quantum wells. A comparison with photo-induced intersubband absorption in GaAs quantum wells as well as the variation of the intersubband absorption with Ge content, layer thickness and doping and the temperature dependence of photoinduced absorption are reported. The normal incidence IR modulation is presented in a second part. It is shown that the index variations associated with free carrier and intersubband absorptions significantly affect the IR modulation. The thermal stability of narrow Si-Si1-xGex quantum wells is investigated in a third section. The interdiffusion leads to a red shift of the intersubband transitions. It is shown that the local variation of the effective mass can enhance the normal incidence intersubband absorption. Finally, intersubband relaxation in doped quantum wells is investigated using a picosecond free electron laser. The subband lifetime is first deduced from the saturation of intersubband absorption vs, pump intensity. In a second step, the subband lifetime is measured directly by time-resolved pump and probe experiments, Both techniques demonstrate that the subband lifetime T-1 is shorter than 1 ps.