Modern science and technology continually demand novel materials with specialized properties to drive groundbreaking innovations. This progress revolves around refining scientific and technological manufacturing processes to explore advanced materials, making them faster, more cost-effective, and of higher quality. Within this context, the field of rare earth (RE3+)-doped glass spectroscopy is rapidly expanding to attain enhanced optical quality materials at an economical scale. This is fueled by the growing interest in their potential applications as materials for lasers, photonics, waveguides, and luminescent devices. This book succinctly summarizes the focus on the preparation, characterization, and the structural and photoluminescence properties of RE3+ ions. The incorporation of these ions into glasses holds significant promise for numerous potential applications in photonic devices.