Unlike thermoplastic polymers, which are processed in the molten state without altering their molecular structure, chemical reactions are involved when utilizing thermosets, for example, as an adhesive or to fabricate composite parts. As a result of these reactions, thermosetting resins form a cross‐linked network and become “set,” that is, they can no longer flow or dissolve. In contrast to cross‐linked elastomers or rubbers, the glass transition temperature (
T
g
) of thermosets is generally above room temperature. They may contain additives such as catalysts to promote or accelerate cure. Many thermosets are used in filled or reinforced form. While epoxy resins are the best‐known thermosets, others include phenolic resins, unsaturated polyesters, polyurethanes, dicyanates, bismaleimides, acrylates and methacrylates. Topics covered in this article include the distinguishing characteristics of thermosetting materials; the more common thermosetting resins, including their cure chemistry; thermoset applications; materials and process characterization; cure, including cure kinetics and cure monitoring; and selected thermoset processes.