The main goal and impact of modern string theory is to provide a consistent quantum theory of gravity. this book provides an updated collection of original new developments and fundamental research in string theory in connection with gravity and physics at the Planck energy scale. Topics treated in this volume by pioneering researchers in the field include: classical and quantum string dynamics in strong gravitational fields, space-time singularities, black holes and cosmological backgrounds; particle and string scattering at the Planck energy scale; string cosmology and its observational consequences; the new features of multistrings and of quantum particle transmutation for strings in curved spacetimes. The book deals with (i) the several new methods developed to solve the highly nonlinear string dynamics in curved spacetimes: approximative perturbative methods, asymptotic expansions, exact local expansions and exact global (over the whole world sheet) string solitonic solutions, (ii) the string energy momentum tensor and the equation of state for the string matter, the stretching of the string size in spacetimes with event horizons and near spacetime singularities, (iii) the canonical and semiclassical quantization of strings in curved spacetimes and the physical effects found for: the mass spectrum, structure of levels, scattering amplitudes, number operator and particle transmutation.