This book presents a comprehensive and self-contained exposition of the mathematical theory of impulsive light-like signals in general relativity. Applications are provided in relativistic astrophysics, cosmology and alternative theories of gravity deduced from string theory. Cataclysmic astrophysical events give rise to impulsive light-like signals which can generally be decomposed into a thin shell of null matter and an impulsive gravitational wave. Several examples are considered in black hole physics, wave collisions and light-like boosts of compact gravitating sources. Graduate students and researchers in relativistic astrophysics, cosmology and string theory will find this book very useful.