However the understanding of black hole thermodynamics in the semiclassical and furthermore in quantum regime has been a very difficult, and still unsolved problem. To explain the situation, it is known that, in statistical physics, entropy counts the number of accessible microstates that a system can occupy, where all states are presumed to occur with equal externally observable classical parameters: mass, electric charge, and angular momentum. All other information about the matter which formed a black hole "disappears" behind its event horizon, and therefore the nature of these microstates is obscure. Thus, the origin of the black hole entropy is not clear. Furthermore, in order to justify the name "entropy", one must to explain also why the sum of the entropy of a black hole and the entropy of its vicinity is a non-decreasing function of time. In other words, why black holes obey the so called "Generalized Second Law of thermodynamics (GSL)".