A microscopic model for the mechanism causing the ferroelectric phase transition in the tetragonal-tungsten-bronze structure ferroelectrics is introduced. It is based on an interaction between the ferroelectric A1 phonon displacement and dynamic local structural instabilities in these materials. The resulting description of the ferroelectric phase transition accounts for its essential experimental features including the absence of a “soft” phonon mode. The model is also used to discuss the unusual superconducting properties of the alkali tungsten bronzes.