This book presents new, promising theories on soft X-ray spectroscopy. As third generation synchrotron light sources have started operating, more precise and sophisticated theories on X-ray spectroscopy are needed. In particular, many-body effects on optical spectroscopy are the hottest topic in solid state physics, since they seem to break conventional mean field pictures completely. Conceptual and instructive theories on X-ray scattering, photoemission, and light absorption spectroscopy for many-body systems are introduced in this book. In addition, many important and lasting experiments are referred to, and compared with the authors' theories.