To the surprise of practically no one, research and engineering on multi polymer materials has steadily increased through the 1960s and 1970s. More and more people are remarking that we are running out of new monomers to polymerize, and that the improved polymers of the future will depend heavily on synergistic combinations of existing materials. In the era of the mid-1960s, three distinct multipolymer combinations were recognized: polymer blends, grafts, and blocks. Although inter penetrating polymer networks, lPNs, were prepared very early in polymer history, and already named by Millar in 1960, they played a relatively low-key role in polymer research developments until the late 1960s and 1970s. I would prefer to consider the IPNs as a subdivision of the graft copolymers. Yet the unique topology of the IPNs imparts properties not easily obtainable without the presence of crosslinking. One of the objectives of this book is to point out the wealth of work done on IPNs or closely related materials. Since many papers and patents actually concerned with IPNs are not so designated, this literature is significantly larger than first imagined. It may also be that many authors will meet each other for the first time on these pages and realize that they are working on a common topology. The number of applications suggested in the patent literature is large and growing. Included are impact-resistant plastics, ion exchange resins, noise-damping materials, a type of thermoplastic elastomer, and many more.