This paper reviews the subject  of size effect on ferroelectric behavior of thin films based on experimental observations and theoretical models. It describes two critical sizes. One that marks the transition from multi-domains to single domain grains and another that indicates the transition from single domain grains to an absence of domains and  ferroelectric behavior. The value of the second critical size may vary, even for the same material, depending on various parameters such as microstructure, composition and stress of the film. Under specific conditions this critical size can be reduced to few nanometers  and practically eliminated, which opens the route for fabrication ferroelectric–based devices in the  nanometer size range.

Examples of BaTiO₃ and copolymer vinyliden fluoride are given for the presence of ferroelectric behavior in few nanometer thick films. The examples also include HRTEM images of nano-domains having a thickness of 1-3 units cells. Their thermo dyamic stability is considered and criterions are established for their formation. The size effect on the transition temperature, Tc, from ferroelectric to paraelectric phase is also considered . It shows experimental evidence for a shift of Tc, as the film thickness decreases to the nanometer size range  and presents two theoretical models to evaluate it.