Composite structures are produced by placing at least two flat, thermoplastically formable plastic materials one above the other and thermoplasticially molding the materials under sufficient conditions of temperature and pressure that molding is accompanied by intimate union of individual layers of the structure. At least one of the plastic materials is based on a polyisocyanate polyaddition product having a density of at least 0.8 g/cm.sup.3 and is prepared by reacting:a) an aromatic polyisocyanate,b) a compound having a molecular weight of about 1800 to 12,000 and containing an average of at least 2.5 isocyanate reactive groups,c) optionally a diamine having a molecular weight of from 108 to 400 and containing two primary and/or secondary aromatically bound amino groups, andd) optionally an aliphatic or cycloaliphatic polyol and/or an aliphatic or cycloaliphatic polyamine having a molecular weight of from 60 to 1799 and which may contain ether groups,at an isocyanate index of from about 60 to 140. The total combined amount of components c) and d) is at least 5% by weight, based on the weight of component b).