Polar oxide surfaces

In the light of recent experimental as well as theoretical studies, we summ arize our present understanding of polar oxide surfaces and examine fundame ntal issues regarding their stability. The focus is on the surface atomic c onfigurations (relaxations, reconstructions, nonstoichiometry, etc) obtaine d under specific preparation conditions and their associated electronic str ucture. We discuss several mechanisms at work on polar surfaces, such as re laxation effects, change of covalency in the surface layers, partial fillin g of surface states, and stoichiometry variations, and try to assess their actual efficiency for cancelling the polarity.