Growth and properties of Si/Ge and Si/SixGe1-x strained layer superlattices are reviewed. The critical thickness of single layers and asymmetrically strained superlattices are determined by LEED and Raman spectroscopy. The importance of strain symmetrization is discussed. Built-in strains are determined by phonon Raman scattering. The effects of strain on the band structure are analysed theoretically. Transport measurements in selectively doped samples lead, in connection with self-consistent subband calculations, to a consistent picture of band ordering. In short period superlattices a quasi-direct band gap semiconductor can be achieved. Zone-folding effects are also observed in the phonon properties of such superlattices. They are discussed both for the acoustical and optical branches.