The concept of fibre-reinforced polymer composites is introduced in this chapter. Most fibre-reinforced composites (FRCs) comprise two constituent materials (known as phases): a fibre phase and a matrix phase. It is noted that thermosetting matrices are far more frequently used for FRCs than their thermoplastic counterparts. An overview of the three most commonly used thermosetting matrices (i.e. unsaturated polyester, vinylester and epoxy) is provided. Available fibre types and forms are considered. The representative mechanical properties for the most common fibre types (glass, carbon and aramid) are presented and compared to traditional materials.