Great progress has been made in the field recently in term of material sensitivity and efficiency under applied electric field. It is undoubtedly a class of material of growing interest. A closely related field is the photorefractive effect in liquid crystals materials, which exhibit attractive perspectives due to their large photoinduced index modulation. This volume gives an in depth review of the present understanding of the fundamental origins of the effect in a variety of materials. All of the materials considered will play a significant role in the development of applications presented in the third volume of this series. The contribution of the material is decisive for new progress in the field of photorefractive nonlinear optics. It is therefore most important to stimulate significant efforts of research on the basic physical phenomena in different materials. These research achievements may contribute to the discovery of a new class of photorefractive material.