This book is dedicated to Professor Leonid V Keldysh. His brilliant contributions to condensed matter physics include the Franz-Keldysh effect, an electron-hole liquid, the nonequilibrium (Keldysh) diagram technique, Bose-Einstein condensation (of excitons) and a ``metal-dielectric'' transition, acoustically-induced superlattices, multi-photon transitions and impact ionization in solids. In many respects, his work influenced and formed the paradigm of modern condensed matter physics. As a result, many famous researchers in the field have enthusiastically provided unique contributions to the book.