In this thesis we study string theory on orbifolds of AdS3. Non-extremal BTZ black holes have been shown to oer a good opportunity to study closed string tachyon condensation, as there are tachyons in the winding sector even in superstring theory. We study extremal BTZ black holes, both M = 0 and M = J from a world sheet perspective. The string spectrum is calculated within bosonic string theory and\ud tachyons are identied within the spectrum. The \ud at space limit of the M = 0 black hole is considered and an extension to superstring theory is discussed. In the\ud second half of the thesis we discuss the self dual orbifold. The self dual orbifold is a simple example of a geometry which contains an AdS2 factor. AdS2 factors also\ud appear in the near horizon limit of extremal Kerr and Reissner-Nordstrom black holes. Using the AdS/CFT correspondence we conjecture that the self dual orbifold\ud is dual to a CFT on two distinct boundary regions and nd evidence to support this statement. We consider asymptotically self dual orbifold spacetimes, one of which\ud is dual to a single copy of the groundstate of the CFT
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