This second volume of the series on photorefractive effects focuses on the most recent developments in the field and highlights the parameters which govern the photoinduced nonlinearity. Besides reviewing conventional electro-optic crystals, this book deals with organic photorefractive materials, giving an in-depth assessment of the present understanding of the effect in a variety of materials. The materials considered in this volume will play a significant role in the development of applications such as presented in the third volume.