I will describe recent studies of bound states of NS5 branes carrying momentum and/or fundamental string charge, in the decoupling limits leading to little string theory and to AdS3/CFT2 duality. This work involves a class of exactly solvable worldsheet models that describe families of BPS and non-BPS black hole microstates. These models have enabled studies of string and D-brane probes of these microstates, yielding insight into their stringy structure in the gravitational bulk description.

The Information Paradox represents a strong consistency challenge for any quantum theory of gravity. The study of black hole internal structure in String Theory offers the potential to resolve this paradox. I will give an overview of recent work on constructing families of smooth horizonless supergravity solutions describing black hole microstates. Where applicable, I will present a holographic description of these solutions. I will also discuss the physics of an observer falling into a black hole.