Details of energy transfer phenomena and scintillation mechanisms in luminescent media excited by ionizing radiation are discussed in this chapter. The sequence of relaxation of electronic excitations is described: creation of electron-hole pairs, energy transfer to emitting centers of interest and quantum efficiency of these luminescence centers. The theoretical limit of the light yield is usually much higher than the measured value. The limiting factors at each step of relaxation are considered in self activated, doped and cross-luminescent scintillation materials. The final stage of luminescence centre excitation mechanism in scintillators has a strong influence on the scintillation parameters. It is discussed in detail here. Finally different examples are given of charge transfer and non-radiative relaxation processes of scintillation centers through their coupling with the crystal lattice.