  We give a broad conceptual review of what we have learned about black holes
and their microstate structure from the study of microstate geometries and
their string theory limits. We draw upon general relativity, supergravity,
string theory and holographic field theory to extract universal ideas and
structural features that we expect to be important in resolving the information
problem and understanding the microstate structure of Schwarzschild and Kerr
black holes. In particular, we emphasize two conceptually and physically
distinct ideas, with different underlying energy scales: a) the transition that
supports the microstate structure and prevents the formation of a horizon and
b) the representation of the detailed microstate structure itself in terms of
fluctuations around the transitioned state. We also show that the supergravity
mechanism that supports microstate geometries becomes, in the string theory
limit, either brane polarization or the excitation of non-Abelian degrees of
freedom. We thus argue that if any mechanism for supporting structure at the
horizon scale is to be given substance within string theory then it must be
some manifestation of microstate geometries.

High Energy Physics - Theory;General Relativity and Quantum Cosmology