Secondary caries, caries surrounding dental fillings, is the most common reason for their replacement, amounting up to 70% of replaced fillings. Bioactive remineralizing composite materials with the ion-releasing ability and the potential to build up destructed tooth structure by imitation of naturally occurring process of mineralization will be the foundation of our work and a strategy for secondary caries prevention. The main shortcoming of the majority of remineralizing composites is their lower mechanical strength and faster degradation in oral environment, mainly caused by the presence of unsilanized, soluble, ion-releasing fillers. In the proposed project, this issue would be addressed by the modification of resinous matrix and/or incorporation of several kinds of bioactive and inert fillers and fibers, including specifically formulated bioactive glass fillers. The latest commercially available materials and polymerization lights would be used for comparison. New experimental composite formulations would be subjected to comprehensive evaluation of their remineralizing potential, mechanical properties, polymerization kinetics, water sorption and solubility, light transmission, temperature rise and bond to hard dental tissues. Most of the planned studies would be conducted during one year so that long-term behavior could be assessed. This should all contribute to better understanding of the composition-structure-property relationship and optimizing composite formulations in further studies, which should bring this promising class of biomaterials a step closer to clinical applicability. The repercussions of secondary caries are physical pain, self-consciousness due to personal appearance and significant financial burden for patients, their employers, health insurance and the state economy. This unwanted series of events could be averted by preventing secondary caries and improving the quality of life of our patients with our bioactive dental composite materials.