The adhesion of glassy or semicrystalline polymers to themselves or to other polymers is often poor. Polymers gain their strength from the mechanical interconnection between different chains. This interconnection can occur either by the use of long chains that entangle with each other or by chemically joining the chains by cross-linking. A strong polymer-polymer interface is crossed by many chains that are mechanically coupled into the material on both sides of the interface. For cross-linked systems these coupling chains must be cross-linked into both sides. If the polymers are not crosslinked, the coupling chains are formed by interdiffusion or welding. Interdiffusion can occur only if the system is heated to a temperature that allows significant chain mobility and if the interconnecting chains are sufficiently miscible in both polymers. For some systems, such as polyimides, the mobility required can either be generated by just part curing the materials before joining or by reversing the completed cure of a thin surface layer before joining and completing the cure. The interconnecting chains do not have to be the same material as the polymers being joined and, for joining dissimilar polymers, can be block copolymers where each block is miscible in just one of the relevant polymers.