This is the final volume of a series devoted to photorefractive effects, photorefractive materials and their applications. Since publication of the first two volumes almost 20 years ago, new and often unexpected effects have been discovered. Theoretical models have been developed, known effects can be finally explained and novel applications have been proposed. For this volume, the editors have invited top experts to reflect on the maturity of the field, assessing progress so far, and predicting avenues of future development. In addition, a series of applications of photorefractive nonlinear optics and of optical data storage are presented in several chapters.