The physical properties of silicon on sapphire (SOS) thin films and their impact on the operation of integrated devices are systematically reviewed. The preparation of the SOS material is described from the fabrication of the sapphire substrate and the silicon film heteroepitaxy up to the .recently proposed regrowth techniques. The microscopic properties and the relevant methods used for inspection are discussed with emphasis on autodoping, stress, lattice defects and interface behaviour. Following the presentation of average transport and recombination parameters, deduced from measurements involving the contribution of the whole film, the non-homogeneity of the S O S film along the epitaxial direction will be outlined. The electrical properties of the silicon-sapphire interface and of the interfacial transition layer are compared to those of the front Si-SiO, interface. Attempts are made to correlate the data derived by various characterisation techniques in order to reveal a coherent image of the Si film volume sandwiched by two very dissimilar interfaces. Finally, the processing and the performance of SOS devices are discussed and favourably compared to the case of bulk Si technology.