AbstractA new class of macromolecules has been synthesized based on a skeletal structure of alternating phosphorus and nitrogen atoms (I). These are the first high polymers with an inorganic backbone to be developed on a broad scale since the discovery of the poly(organosiloxanes) in the 1940's.The preferred synthesis route makes use of highly reactive polymeric intermediates (I, X = C1 or F) used as substrates for halogen replacement by organic nucleophiles. Structural diversity is achieved either by changes in the nucleophile or by chemical modification of the side groups after they are attached to the backbone. Using these principles, a wide range of different polymers has been prepared ranging from elastomers to carrier molecules for steroids, transition metals, metalloporphyrins, or carboranes. Biodegradable and biocompatible macromolecules have also been synthesized. The critical role of small molecule model reactions is stressed, and examples are given of recently discovered organometallophosphazenes that are expected to have a profound influence on future polyphosphazene syntheses.