A new class of insulating solids was recently discovered. When irradiated by a few visible photons, these solids give rise to a macroscopic excited domain that has new structural electronic orders quite different from the starting ground state. This occurrence is called "photoinduced phase transition," and this multi-authored book reviews recent theoretical and experimental studies of this new phenomenon. Why and how do photoexcited few electrons finally result in an excited domain with a macroscopic size? How is the resultant photoinduced phase different from the ordinary thermal-induced...