Polyethylene terephthalate (PET) is an aliphatic-aromatic and semicrystalline thermoplastic polyester of prime commercial and industrial importance. Namely, PET is a very important industrial polymer due to its excellent properties such as processability, chemical resistance, high tensile impact strength, high thermal stability and clarity. Chapter One summarises the synthesis and physicochemical properties of PET. In Chapter Two, the authors review the frequency-dependent parameters of the PET substrate, developments of the flexible RF electronics on PET substrate, and the challenges and potentials of RF applications using flexible electronics fabricated on PET films. Chapter Three presents a background of the current state of knowledge with respect to PET recycling.