We briefly introduce black holes in the theory of general relativity
and study their thermodynamic properties. We then move to
supersymmetric black holes, which are solutions to superstring
theories. As an example we concentrate on the D1-D5 brane system in
five dimensions, in the context of compactifications of type IIB
superstring theory on a 5-torus. We compute the macroscopic
(Bekenstein-Hawking) entropy of the corresponding black hole. We then
give a microscopic interpretation of this result in terms of string
microstates, following the work of Strominger and Vafa. We end by
mentioning some current open problems in black hole physics.
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