Internal Structure of Black Holes

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