Phosphazene polymers consisting of a backbone of alternating phosphorus and nitrogen atoms with two substituents at each phosphorus comprise a remarkably diverse class of polymers both in terms of composition and range of properties. While literally hundreds of these polymers with P–N and P–O bonded groups on each phosphorus atom have been prepared by the ring-opening polymerization-macromolecular substitution of [Cl2PN]n, the diversity and potential applications of this class of polymers has greatly expanded with the introduction of complementary condensation polymerization methods. This chapter presents the historical development of synthetic pathways to polyphosphazenes by condensation polymerization, including the thermal polymerization, the ambient temperature living cationic polymerization, and the ambient temperature phosphite-mediated chain-growth polycondensation of N-silylphosphoranimines. These methods produce polyphosphazenes with directly P–C bonded groups at phosphorus and, via the living polymerization approach, affords well-defined polymers with controlled molecular weights and narrow molecular weight distributions. Properties of these systems can be adjusted and tuned through post-polymerization modification reactions at side groups or through the end group functionality in the living systems, thus providing new materials with side group functionality, branched-, grafted- and block-copolymers, and novel supramolecular structures.