Unsaturated polyester resin (UPR) is a relatively low cost and durable thermosetting plastic made up of two parts: liquid resin and catalyst (hardener). The principal difference between these systems and thermoplastic polyesters, such as polyethylene terephthalate (PET), is the inclusion of double bonds in the polymer chain, which allows for permanent cross-linking and the formation of a rigid thermoset structure. This has many advantages: the resin starts off in liquid state and so suitable for casting, pouring, coating, composite laminating and so on; very good resistance to chemicals and weathering; and high adhesive strength with excellent wetting (ability to coat a solid surface, like glass fibre). However, on is own it is quite brittle and so tends to be used in combination with fibre or mineral reinforcement. Also, as a thermoset, it is not practical to recycle. This problem is especially acute in the production of wind turbine blades, where thousands of tons of this material are being consumed each year and will inevitably end up in landfill.

Unsaturated polyester resin contains some harmful chemistry, such as individual ingredients like styrene, a known carcinogen, as well as catalysts like methyl ethyl ketone peroxide (MEKP) that can cause skin damage and blindness.

There are two main families: ortho (based on phthalic anhydride) and iso (based on isophthalic acid). Ortho types are the most common and used in everything from boat hulls to bathtubs and countertops. Iso types are higher priced and higher performance. They are used in applications that require higher levels of scratch resistance, clarity and weather resistance. For example, iso resins are used in gel coats, which provide the outer decorative and protective layer on FRP structures. It is painted onto the mould before the fibreglass is applied behind. So when demoulded, it becomes the visible layer.