The idea of actively using the power of quantum mechanical principles and special entangled states has fueled a revolution in the computing and communications sciences over the last decade. This book presents the historical development and state-of-the-art of quantum cryptography in a comprehensive work representing the collaboration of the most active researchers in the field. Following the leap from research into practical application, the authors cover development of basic physics principles and entangled states generation and demonstrate quantum cryptography with weak coherent states, with entangled photons, with continuous variables, in fiber communication lines, and in free space.