Photoluminescence has been observed from Si~ _xGex alloy layers, superlattices and non-periodic multilayers where x was varied from 0 to 0.6. A p-type Si082Ge0 is alloy layer 200 nm thick, grown by molecular beam epitaxy (MBE), has been fabricated into a mesa diode which operated as a light-emitting diode, emitting at 1.4/~m at temperatures up to 80 K. This diode was selected from a series of heterostructures which exhibited intense photoluminescence with internal quantum efficiencies in the range 1%-10% at low temperatures. Photoluminescence in the wavelength range 1.2-1.7 /zm has been observed from thick (100-200 nm) Si~ _xGe~ alloys and Sil _xGex-Si strained layer superlattices with a range of dimensions which was large compared with the unit cell; i.e. where Brillouin zone folding effects were negligible. The intense Sil -xGex alloy photoluminescence peak had a halfwidth of about 80 meV and the peak energy was found to shift consistently and predictably with the germanium fraction. Photoluminescence peak energies at 4.2 K varied from 620 to 990 meV for germanium fractions where 0.53 > x> 0.06. The photoluminescence and electroluminescence peaks were consistently about 120 meV below the established bandgap for tetragonally strained Sil-xGex. In general, Sil-/Gex strained layers grown at low temperatures typical of MBE (below 500 °C) exhibited low photoluminescence intensity. However, post-growth annealing in the 500-700 °C temperature range enhanced luminescence efficiency by up to two orders of magnitude. A comparison is also made of the broad intense photoluminescence observed from MBE material with the weak near-band-edge spectra obtained from Sil _xGejSi grown by low temperature chemical vapour deposition.