This book is concerned with the underlying science and design of laser materials. It emphasizes the principles of crystal-field engineering and discusses the basic physical concepts that determine laser gain and nonlinear frequency conversion in optical crystals. A concise review of the essential underlying science is presented, and the predictive capabilities of crystal-field engineering are developed to show how modification of the symmetry and composition of optical centres can improve laser performance. Applications of the principles of crystal-field engineering to a variety of optical crystals are also discussed in relation to the performances of laser devices. This book will be of considerable interest to physical, chemical and material scientists and to engineers involved in the science and technology of solid state lasers. It will be used by senior undergraduate and postgraduate students as well as by established scientists.