Point Defects in LiNbO3.- General Introduction to Photorefraction in LiNbO3.- Photorefraction in LiNbO3 Crystals with Different Stoichiometry and/or Doped with Optical-Damage-Resistant Impurities.- Problem of a Non-Erasable Photorefractive Hologram.- Properties to Characterize Undoped and Optical-Damage-Resistant LiNbO3 Crystals.- Polarization Reversal and Ferroelectric Domains in LiNbO3 Crystals.