What is inside a black hole?

In the conventional picture of a black hole we cannot see the states which account for its entropy, and Hawking radiation is unable to carry out the information of the state of the hole. We outline some computations in string theory which suggest that bound states expand to a size determined by their degeneracy. Thus the state information of a black hole would be distributed throughout the interior of the horizon, and Hawking radiation can 'read' this information. No individual microstate has a horizon, rather the horizon arises upon coarse graining as a boundary of the region where the microstates differ. (c) 2005 Published by Elsevier SAS on behalf of Academie des sciences.