This book deals with the so-called non-steady-state photo-EMF effect and its applications for material characterization. The first part of the book represents theory of the non-steady-state photo-EMF effect. The second part of the book describes application of the non-steady-state photo-EMF effect for investigation of wide-bandgap semiconductors and nanostructured materials including photorefractive sillenites grown in the air and argon atmosphere, nanosized GaN and Se within porous matrices, SiC, AlN and gallium oxide crystals. This book has been written to be accessible to the specialists working in solid state and semiconductor physics. References to over 150 original papers represent the personal selection of the authors.