The book presents the current state of studies of point defects, both intrinsic and extrinsic (impurities, radiation centers, etc.) in LiNbO3. The contribution of intrinsic defects to photoinduced charge transport, i.e. to the photorefraction, is explained. The photorefractive and optical properties of LiNbO3 crystals with different stoichiometry and of those doped with so-called 'optical-damage resistant' impurities controlling the intrinsic defect structure are described in detail. Applications are the problem of non-erasable recording of photorefractive holograms in LiNbO3 and the current situation of studies in the ferroelectric switching and domain structure of LiNbO3 as well as the creation of periodically-poled structures for the optical frequency conversion.