Thermosetting polymers, also referred to as thermosets or duromers, are plastics that harden through a chemical reaction. They usually react by polyaddition and crosslink to form a closely interlinked lattice of macromolecules, which gives the thermoset its high mechanical strength. Since this kind of hardening, or curing, changes the structure of the molecule, it is an irreversible process.

Thermosets have no softening range; the material retains its strength even at high temperatures and has excellent heat resistance. Thermosets are characterised by their hardness and rigidity. Their low coefficient of thermal expansion ensures high dimensional stability. The behaviour of thermosets in fire is also advantageous.