Rare-earth (RE) doped nanomaterials have already proven promising materials for advanced materials and technologies including optics, lasers, catalysts, alloys, magnets, electronics, lighting, bioanalyses, imaging etc. because of their outstanding properties such as extremely narrow emission bands, long lifetimes, large strokes shifts, photostability and absence of blinking. The efficient of RE doped phosphors is found to be controlled by tuning nonradiative relaxation pathway which eventually controls by tuning crystal phase of host, lattice vibration, concentration of dopant etc. Cross relaxation nonradiative decay due to concentration quenching can be manipulated by controlling dopant concentration. This review article highlights the optical properties of Eu³⁺ ion in various hosts such as fluoride, phosphate, silica, semiconductor, oxyhalide, vanadate, molybdate and tungstate because of its importance for potential applications. It is important to know how the host environment influences the radiative and nonradiative relaxation which eventually controls the overall photoluminescence properties. Of particular attention is how the optical properties of Eu³⁺ ion vary with changing the host environment with the anticipation that such knowledge will permit us to construct efficient nanomaterials. Finally, a tentative outlook on future advances of this research field is given.