We present a phenomenological model of ferroelectricity that quantitatively accounts for both displacive like and order-disorder like properties of pure perovskite ferroelectrics. We further show that this model also accounts for the dielectric and phase transition properties of incipient ferroelectrics doped with off-center impurities. Finally, we present a microscopic model that quantitatively shows that the off-center displacements in a number of perovskites is a manifestation ground state polarons. ©2000 American Institute of Physics.