Lithium niobate, LiNbO , is an oxide ferroelectric with various kinds of pro- 3 nouncedphysicalproperties. Thisversatilityhaspromoteditscareerinscience anddevices. Ithasbeenparticularlyfruitfulintheopticalregime,wheremany e?ects have been found in LiNbO and devices introduced using it as a host. 3 One of the few big drawbacks, namely the low level laser damage threshold based on photorefraction due to extrinsic defects was discovered very early. A relatively new topic, not involved so far in any general description, is a fundamental dependence of the optical properties of LiNbO on intrinsic de- 3 fects. Their importance has been realised out due to the development of varies growthtechniquesintherecentpast. Theprogressinthegrowthandstudiesof LiNbO crystals with di?erent composition, particularly almost stoichiomet- 3 ric ones, has revealed a signi?cant and sometimes decisive role of the intrinsic defects. For example, the photoinduced charge transport, and therefore the photorefractive properties governing the recording of the phase gratings in LiNbO , are strongly controlled by the content of intrinsic defects. The re- 3 cently found impact of intrinsic defects on the coercive ?eld in LiNbO is 3 of fundamental importance for the creation of periodically poled structures (PPLN) aimed at the optical-frequency conversion in the quasi-phase mat- ing (QPM) mode of operation. As a consequence of these results, an idea of the intrinsic defects in LiNbO has been developed during the last decade 3 and involves microscopic studies on defects, photorefraction and ferroelectric switching using spectroscopic and structure methods.