The linear electro-optic effect which exists in acentric crystals was first observed in quartz by Rontgen and Kundt in 1883. In these first measurements it was not clear, whether the electro-optic effect observed was due to a direct field induced change of refractive indices or a photoelastic effect due to piezoelectrically induced strain. This question was answered by Pockels in 1893 when he investigated the electro-optic effect in quartz, tourmaline, potassium chlorate and Rochelle salt.1 In his detailed study Pockels demonstrated the existence of a direct effect and characterized the linear electro-optic effect in crystals of various point symmetry using either the applied electric field of dielectric polarization as driving terms. Pockels also pointed out for the first time that Rochelle salt shows some peculiar behaviour upon reversal of the electric field direction, e.g. the electro-optic effect was not reversed by reversion of the field directions as should be the case in piezoelectric non-ferroelectrics. This observation is certainly connected with ferroelectricity. A better understanding of the interrelationship between ferroelectricity and electro-optic effects was obtained in the electro-optic experiments reported by Zwicker and Scherrer in 1944.2 They observed that the electro-optic coefficients of KH2PO4 and KD2PO4, based on electric fields, was proportional to the dielectric constant, exhibiting a Curie-Weiss behaviour as a function of temperature. The electro-optic coefficient based on dielectric polarization was found to be the same temperature-independent constant for both crystals. In addition the electro-optic effect was shown to be a very graphic means of observing the spontaneous polarization, hysteresis, Barkhausen jumps etc. in the ferroelectric phases of the materials. From the experiments of Zwicker and Scherrer one could conclude, that large electro-optic effects would be observed in materials with large dielectric constant and spontaneous polarization.