An interpenetrating polymer network (IPN) is a material containing two polymers, each in network form. In biomaterials used in dentistry, the IPN-like nanostructures are used in denture base polymers, denture teeth and fibre-reinforced composites. IPN structures provide the specific and desired properties for the resin system prior to and after polymerization. At adhesive interfaces, IPN polymers and composites provide good interfacial adhesion for adhering and veneering resin composites based on the so-called secondary-IPN bonding formation due to swelling of the IPN nanostructure. Good interfacial adhesion is a requirement for the success of modern adhesive dentistry. (C) Koninklijke Brill NV, Leiden, 2009