The pioneering work of Bekenstein and Hawking in the 70's produced a universal area law for black hole entropy valid in the infinite size limit.  Quantum corrections to the gravitational action induce finite size corrections to the black hole entropy.  I shall report on progress in the computation of the exact quantum entropy of supersymmetric black holes in supergravity, using localization techniques.  In simple examples in string theory, one has a solvable dual microscopic description as an ensemble of microscopic excitations. I shall describe how the gravity functional integral leads to the microscopic integer degeneracies of this black hole, and its associated number theoretic properties.