The recent progress in the field of modulation-doped Si/SiGe heterostructures is reviewed. Low temperature mobilities in n-channel structures are now close to 1,000,000 cm(2)/V s, and maximum oscillator frequencies of 120 GHz for n-, and 85 GHz for p-MODFETs were reported for room temperature operation. Progress has also been made toward a production compatible realization of strain-adjusting buffer layers by introducing a high concentration of implanted hydrogen close to the Si/SiGe interface prior to thermally driven relaxation. New simulations in connection with high-quality SiGe bulk crystals shine new light on the important role of alloy scattering in Si1-xGex and Si1-yCy channels. The discovery of a kinetic growth instability on the Si(001) surface may have important consequences for interface roughness scattering.