An interpenetrating polymer network, IPN, is a combination of two polymers in network form, at least one of which is synthesized and/or cross‐linked in the immediate presence of the other (L. H. Sperling, Interpenetrating Polymer Networks and Related Materials, Plenum Press, New York, 1981). An IPN is distinguished from other multipolymer combinations, such as polymer blends, blocks, and grafts, in two ways: (
1
) an IPN swells, but does not dissolve in solvents; and (
2
) creep and flow are suppressed.
This article discusses synthesis, morphology, glass transitions, and mechanical behavior of IPNs, and outlines known and proposed applications of these materials. Some applications of note include sound and vibration damping, biomedical applications, natural products and renewable resources, tough and impact‐resistant materials, etc.