INTERNAL STRUCTURE OF BLACK-HOLES

We present a number of qualitative arguments which strongly suggest th at extremal dyonic black holes in the 4-dimensional low energy, classi cal field theory limit of toroidally compactified heterotic string the ory represent largely degenerate classes of states in the quantum theo ry. We propose a simple expression for the full non-perturbative degen eracy, which contains no free continuous parameters and reproduces the Bekenstein-Hawking S = A/4G(N) in the large-area limit (with the 1/4 arising from microphysics). We sketch the elements of a physical pictu re leading to this expression: the holes support much hair, such that in counting it we are led to an effective string theory, and a matchin g condition whose solutions give the degeneracy.