Rare earth (RE)/transition metal (TM) ion-incorporated luminous materials with up/downconversion phenomena have found extensive use in biomedical disciplines, photovoltaic/solar cells, optical thermometry, sensing, and white LEDs (wLEDs). The purpose of the enhanced research on these materials has been to investigate their luminous qualities for the previously specified uses. It is critically necessary to clarify how to accurately design the right luminous material for a certain application, assess the sufficiency of the synthesized phosphor, and then achieve focused irradiation for its desired and useful use. To increase the efficiency of solar cells, a variety of materials with up-/down-conversion are being researched. To comprehend the necessity of creating new materials to improve the quantum efficiency of commercial white light-emitting devices, several materials utilized for wLEDs and solar cells are discussed. This chapter provides a comprehensive overview of recently investigated phosphors doped with various RE/TM ions to gain a deeper understanding of these materials and to address the problems related to light-emitting diodes and photovoltaics.