Poly-ether-ether-ketone (PEEK) is a semicrystalline thermoplastic in the polyaryletherketone (PAEK) family, with excellent mechanical and chemical resistance properties that are retained to high temperatures. It is extensively used in the aerospace, automotive, teletronic, and chemical process industries. It is highly resistant to thermal degradation as well as attack by both organic and aqueous environments. It is attacked by halogens and strong Lewis acids as well as some halogenated compounds and aliphatic hydrocarbons at high temperatures. The Young's modulus is 3.6 GPa and its tensile strength 90 to 100 MPa. PEEK has a glass transition temperature at around 143°C and melts around 343°C. PEEK's mechanical properties at elevated temperatures have led to it being used in at least two varieties of Reprap extruder as thermal insulation. PEEK is considered an advanced high performance plastic used in medical implants. A disadvantage is the high price, which limits its application to high value items. This book is the collection of several researches on PEEK based composites done by my research group.