Reflection High Energy Electron Diffraction (RHEED) is the most convenient method of monitoring the structure of films grown during molecular beam epitaxy (MBE). Information can be obtained about the growth mode, types of disorder and symmetry of the surface unit cell. Time dependent changes of the intensity can be related to the growth rate and surface kinetic processes. It may be possible to add a novel form of photocurrent spectroscopy to RHEED equipment which will permit the band gap and quantum well layer thickness to be measured.