Solid-state laser and luminescent materials activated by rare-earth or transition metals ions are widely used for solid-state lasers, luminescent lamps, flat displays, optical fibre communication systems, and other photonic devices. The unique solid-state electronic properties enable the activators in solids to emit photons efficiently in visible and IR regions. The rapid advances in both materials science and optoelectronics, particularly, the development of new methods of material synthesis and device fabrication, have been stimulating the growing interests in the deep insights of spectroscopic properties of solid-state laser and luminescent materials. This book brings together essential and practical knowledge of spectroscopic physics. This includes, atomic spectroscopy, mathematical theory, rare earth ions in materials, light emission and absorption, spectral properties, non-radiative transitions and energy migration.