NANOCRYL® D 321 is a versatile dispersion of colloidal silica in a blend of difunctional methacrylate monomers for the use in dental applications. The silica phase consists of surface-modified, synthetic SiO2-nanospheres of very small size (Ø 20 nm) and narrow particle size distribution. Despite the high SiO2-content of 50 %, Nanocryl® D 321 is highly transparent and shows no sedimentation due to the agglomerate-free dispersion of the nanoparticles in the acrylate.