String Theory is a promising theory of quantum gravity, unifying all known forces. However, it is notoriously hard to extract testable predictions from it. Black holes are ubiquitous in string theory (and any theory of General Relativity (GR) for that matter). Their study offers a shortcut for successfully testing the validity of string theory. This projects pursues a theoretical test of string theory, through the study of black holes. In classical GR, black holes are solutions with enigmatic properties, such as a spacetime singularity. When coupling them to quantum fields, there appear questions that GR cannot explain: the information paradox, black hole entropy.In this project, we will answer these issues by studying black hole microstates. Their existence would resolve the black hole singularity, solve the information paradox and explain black hole entropy. For supersymmetric black holes, which are rather unrealistic, there has been quite some success in constructing these microstates, but not nearly enough to solve all black hole problems. We plan as a first step in the string theory test to systematically construct (1) new supersymmetric black hole microstates, and (2) take the story further to more realistic, non-supersymmetric radiating black holes. A later stage of this research will be to determine their properties. Additionally, the implications on cosmological singularities will be intitiated, with the expertise of the GRAPPA Institute (Gravitational AstroParticle Physics Amsterdam), affiliated with the ITFA, training the researcher in a new field (cosmology).In addition, the researcher will supervise master students, coach graduate students and lecture graduate courses in an international school. Furthermore, he will present his work in leading international conferences, and develop research management skills. This complete and intensive training will greatly enhance the researcher's abilities to lead a research group in the future.