This chapter presents a comprehensive analysis of luminescenceLuminescence decayLuminescence decay lifetime curves as a powerful tool to investigate radiative and non-radiative processes in rare-earth-doped materials. By examining the temporal evolution of excited-state populations, decay lifetime measurements enable the determination of intrinsic radiative lifetimesRadiative lifetime, and the influence of multipolar energy transferEnergy transfer mechanisms. Monoexponential decay curves are indicative of isolated ions with minimal interaction, allowing the direct evaluation of radiative properties. In contrast, non-exponential decay behavior arises from ion–ion interactions, such as cross-relaxation, energy migration, and cooperative sensitization, which become dominant at higher dopant concentrations or in disordered host matrices. The chapter introduces analytical models, such as the multi-exponential fit, stretched exponential function, and the Inokuti–Hirayama model, which describe complex decay profiles and reveal the physical nature of donor–acceptor interactions. These models are essential for quantifying non-radiative transfer rates and identifying the dominant interaction mechanisms, such as dipole–dipole or dipole–quadrupole coupling. Overall, decay lifetime analysis provides fundamental insights into the photophysical behavior of rare-earth ionsRare-earth ions and serves as a key diagnostic method for optimizing optical materials used in lasers, phosphors, photovoltaics, and up-conversionUp-conversion devices.