The main task of a dental filling material is to permanently seal the dentinal wound and to restore the anatomy. In particular with respect to the former requirement, glass-ionomer cements exhibit, thanks to it's ingenious composition, unique properties, which allows it to react on external information in such a way that the cement automatically and adequately fulfills it's function as a proper restorative material. Because of it's setting mechanism, glass-ionomer cement restorations have a better perspective to seal the tissue than amalgam or resin composites do. As resistance to wear of glass ionomers is inferior to resin composites it proves to be a better dentin replacement, than a substitute for tooth enamel. Restorations made in a combination of glass-ionomer cement and resin composite, the so called sandwich restorations, have shown to function excellently, both at the tooth interface as well as the outer surface. As any leakage still cannot be excluded, the fluoride release at the margins of the restoration forms a wellcome contribution to the prevention of secondary caries. Once more, glass-ionomer cement shows to be intelligent by automatically activating an advantageous process when the environment requires it.