Isolated horizons and black hole entropy

The aim of this work is to investigate symmetry based approaches to the problem of black hole entropy. Two sets of symmetries are presented. The first one is based on the new framework of isolated horizons which has emerged recently and which provides a local description of black holes. We describe this framework in 2+1 dimensions and then use it to investigate the question of black hole entropy. We show that the natural symmetries of isolated horizons do not suffice to explain the entropy of black holes. We then turn our attention to a different set of symmetries who are distinguished by the fact that they are only defined in a neighborhood of the horizon and do not have a well defined limit to the horizon. It is then shown that these symmetries provide an explanation of the black hole entropy. We then consider the significance of the results obtained for the search of a theory of quantum gravity.