Prepreg is the name given to composite reinforcement materials, such as carbon fibre, that have been preimpregnated with activated resin. The resin system used is almost exclusively epoxy resin, which has already been mixed with its hardener at the manufacturing stage, before being impregnated into the dry reinforcement fabric to make the ‘prepreg’ reinforcement.

Generally, the resin matrix in prepregs is partially cured for ease of handling and is stored in a cool place to prevent complete polymerization. Prepreg is heated in an oven during manufacture to achieve full polymerization. The reinforcement in a prepreg can be unidirectional fibers or a fabric.

The most common way to make a composite part from prepreg reinforcement is to layer the uncured prepreg reinforcement into a mould, vacuum bag the mould and laminate and then cure it in an oven.