Recent advances in mobile telecommunications and satellite broadcasting have created the need for new frequency channels. This fact is stimulating the progress in microwave integrated-circuit technology. The miniaturization and optimization of such circuits create demands for new components, which have to be compact, reliable and of low cost. Microwave dielectric ceramics represent such a group of components. The existing applications of the sillenites, in particular on B12SiO20 (BSO), are mainly in the fields of electro-optics, acoustics and piezoelectics. However, with the discovery of the BSO microwave dielectric properties, a potential application of this material as a dielectric in electronics technology has been considered. Therefore, the scientific work of this monograph is focused on the preparation of BSO thin films using the sol-gel method in order to follow the miniaturization demands for electronic devices. The monograph covers topics from basic sol-gel theory to its application in multi-component thin-film systems. Areas in which particular interest is concentrated include acid sol-gel chemistry, the influence of additives in sol-gel thin-film formation, the control