This chapter examines the following topics: review of thermoset and thermoplastic composites, engineering thermoplastic composites, and high-performance thermoplastic composites. As discussed briefly in previous chapters, all resins are either thermosets or thermoplastics. The pre-cured thermosets are relatively low-molecular-weight resins in which bonds are caused to form between the molecular chains during molding — a process called crosslinking or curing. Crosslinking forms a polymer network of extremely high molecular weight, which results in significant changes in many properties. The thermoplastics, which are not crosslinked, have molecular weights higher than the uncured thermoset resins but not as high as the crosslinked (cured) thermosets. Thermoset polymers are formed, therefore, when crosslinks are formed. The world of composites can be separated into two groups by resin type, thermosets and thermoplastics, and then into two additionalgroups, the engineering or industrial composite materials, and advanced or high-performance composite materials. High-performance thermoplastics are occasionally used in non-reinforced applications where their high strength, high stiffness, solvent resistance, low flammability, and thermal stability are key properties.