This book discusses the spectral properties of solid-state laser materials, including emission and absorption of light, the law of radiative and nonradiative transitions, the selection rule for optical transitions, and different calculation methods of the spectral parameters. The book includes a systematic presentation of the authors' own research works in this field, specifically addressing the stimulated nonradiative transition theory and the apparent crystal field model. This volume is helpful resource for researchers and graduate students in the fields of solid spectroscopy and solid-state laser material physics, while also serving as a valuable reference guide for instructors and advanced students of physics.