Black hole entropy and string theory

Several aspects of black hole entropy in string theory are discussed. It is explained why the geometric entropy in two dimensional noncritical string theory is nonperturbatively finite. Then the philosophy of regarding massive string states as black branes is explained and how the Beckenstein-Hawking entropy for extreme BPS black holes may be understood as coming from degeneracy of string states. This is then discussed in the context of D-strings in Type IIB superstrings. Non-BPS excitations of D-strings and their entropy are described and it is explored that their decay may follow Hawking radiation. For these D-strings and other D-branes the entropy and temperature are consequences of the physical motion of stuck open strings along the D-brane and this leads to a simple space-time interpretation. Finally it is considered that the horizon may be itself regarded as a D-brane. (author)