The interest on poly(organophosphazenes) dates back to 1965, the official date of the synthesis of linear and soluble polydichlorophosphazene, and moved rapidly toward the preparation and characterisation of a very large variety of macromolecules showing different structures and potential and practical use in different fields. Nowadays, polyphosphazene substrates occupy an important position in the panorama of inorganic-organic materials, somewhere at the borderline among the inorganic, organometallic and organic chemistry, and among polymeric, ceramic and metallic materials. Furthermore, theoretical aspects on their structures are still of importance in the scientific debate and their intriguing characteristics deeply investigated. The book opens with a general introduction where the background, developments and future perspectives of poly(organophosphazenes) are presented (Chapt. 1). It divides successively in two parts covering different research areas.