In this Thesis, we study black holes and their microscopic properties in extensions of General Relativity that arise as low-energy limits of String Theory. In order to address how information is released from black holes during evaporation we make use of quantum information techniques and study information release from qubit systems. We then introduce a general framework to capture the Hawking evaporation process and deduce the constraints unitarity puts on the evolution. par This makes the statement of information loss in black hole evaporation more precise and supports the claim that the horizon has to be replaced by a structure, or fuzzball, that carries information about the black hole microstates. This immediately raises the question of what this horizon-scale structure is? We address this question in the context of Supergravity. We systematically construct a family of microstates of near-extremal black holes, by placing metastable supertubes inside certain scaling supersymmetric smooth microstate geometries. These non-extremal fuzzballs differ from the classical black hole solution macroscopically at the horizon scale, and for certain probes the fluctuations between various fuzzballs will be visible as thermal noise far away from the horizon.