The information revolution of this century was brought about by photonics based on simple and common materials, such as glasses and semiconductors. Although semiconductors will continue to be of central importance in the present century, glasses will also be of great importance in future photonic devices since they have various advantages over other materials in homogene­ ity, transparency, easy fabrication and excellent solvent properties. This book focuses on selected topics which are new and of fundamental importance in the applications of active glasses in photonic devices based on our research concept called "induced structure. " The book originates from a final report of the Hirao Active Glass Project (HAP), Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Corporation (JST). Most of the reports in the book concern glasses under the action of very strong electromagnetic fields such as that induced by femtosecond lasers. They include creation of induced structures in glasses, analysis of induced structure, and some functional devices using active glasses. This book is designed to provide graduate students and new researchers with an introductory review of the recent developments in the field of photonic materials. The reader will benefit from an overview of the latest results in the applications of active glasses in photonic devices and from a wealth of knowledge covering most basic solid-state physics, chemical physics, and electronic engineering.