This book focuses mainly on the spectroscopy of laser materials, physics of laser materials, laser crystals and laser glasses. The spectroscopic and laser properties of rare earth and transition metal ion-doped solid state materials are systematically described based on modern quantum optics. The aim of this book is to relate the laser and spectroscopic properties to the structure and chemical composition of materials. It emphasises the nonlinear optical effects in laser materials, which are widely used in high power laser systems. The development of advanced solid state laser devices depends greatly on new laser materials. Much progress has been made recently in the development of new laser materials, and this is summarized in the book.