ADVANCED MATERIALS BASED ON EPOXY RESINS 2. INTERPENETRATING POLYMER NETWORKS Interpenetrating polymer networks (IPNs) are a special type of polymer blends, constituted by combination of two or more network polymers, synthesized in juxtaposition. The synthesized IPNs were carried out in order to improve the material properties of many widely used polymers. Although several papers on IPNs have already described the use of polyurethane (PU) based compositions, the importance of PU-epoxy IPNs needs special emphasis. This paper is a review with some results regarding PU-epoxy IPNs, pointing out own studies on synthesis and characterization of PU-modified epoxy resin semi-IPNs.