The interest on cyclophosphazenes dates back to the mid 1800, and research on these compounds expanded very rapidly over the time. Initially considered mostly as model compounds for the corresponding high molecular weight polymers, they started to assume an increasingly high importance in the middle of the sixties, covering several potential applicative domains as hydraulic fluids and additives, cores for star polymers or dendrimers, starting substrates for supramolecular chemistry, support for crown-ethers, coordination metal complexes, metallacyclophosphazene derivatives, nanostructured materials, cyclic compounds with P-C bonds. This book present chapters on each one of these aspects and also on the spectroscopic properties of both linear and cyclic phosphazene derivatives.