The main goal and impact of modern string theory is to provide a consistent theory of gravity. This book provides an up-to-date understanding of the latest developments and current problems in string theory as applied to gravitation and physics at the Planck energy scale. It also discusses fundamental problems of quantum gravity in a modern context, independent of strings or any other models. The emphasis is on the mutual impact of string theory, gravity and cosmology. <br/> The most relevant new physics provided by strings concerns the quantization of gravity and so we must at least understand string quantization in curved space-times. Besides their obvious relevance in classical gravitation, curved space-times are also important at energies of the order of the Planck scale, where gravitational interactions are at least as important as the rest, and so can no longer be ignored. <br/> The editors have taken particular care to provide the bases of the different lines of research that are currently in contention. The reader is given an excellent opportunity to learn about the real state of the discipline, and to learn it in a critical way.