A survey is given of a variety of IR spectroscopy studies on p-type Si/SiGe quantum wells, Valence-band intersubband absorption experiments were performed on a large number of samples with different parameters and explained quantitatively by a self-consistent Luttinger-Kohn-type calculation. The dependence on polarization of the absorption was analyzed. On the basis of these quantum wells, mid-IR detectors were fabricated and characterized in terms of responsivity, dark current and detectivity. In asymmetric, compositionally stepped quantum wells, second-harmonic generation of CO2 laser radiation was observed.