Black hole entropy

According to the generalized second law of thermodynamics, the quantity that never decreases in irreversible processes involving gravity is the sum of the entropy content of ordinary matter plus the entropy provided by the black holes. As the entropy of matter has a statistical description in terms of microscopic degrees of freedom, it seems that the entropy of black holes also should have such a description. In this contribution, several proposal for address this problem are reviewed, including quantum fields theory, strings and the very recent holographic ideas, resting on the correspondence between anti-de Sitter space and conformal field theories.