This monographdeals with metastable states in amorphoussemiconductors ma- rials which lack long-range periodicity in the atoms positions, which are in th- modynamic nonequilibrium and which, in addition, have several metastable states. Thesestates giverise tovariouspropertiesandeffects namelya widerangeofp- toinduced changes and high photosensitivity and X-ray sensitivity that are unique among solid-state semiconductors.Historically, amorphousselenium and seleni- based materials have played an important role in physics and technology, and they continue to do so. In these materials there exist...