The emission spectrum of Tb3+substituted in YAG to various concentrations (xin Y3−xTbxAl5O12ranges from 3×10−5to 3×10−1) has been measured on samples prepared by two different methods. These methods are powder preparation by coprecipitation and growth of crystalline layers by liquid‐phase epitaxy. The spectrum consists of two groups of lines, one group around 550 nm originating from the5D4level and one group around 450 nm from the5D3level. The intensity of the two groups of lines depends on the method of excitation. In the case of UV excitation (&lgr;=254 nm) the emission intensity for low Tb concentrations is mainly governed by the absorption of the incident radiation. With electron excitation, the energy loss of incoming electrons to ‘‘killer sites’’ in the lattice is found to influence the emission. The ratio, however, of5D4to5D3intensity is independent of the excitation mode. This ratio tends to a constant value at low concentrations and increases strongly with increasing concentration. These effects are explained by feeding of the5D3and5D4states from 5dlevels, followed by cross relaxation, i.e.,5D3to5D4excitation transfer.