Some of our recent spectral and energy transfer results on Tm3+-based 2 micrometer solid state laser systems are presented. Based on experimental data, with an improved algorithm and parameterized crystal field calculations, complete energy level diagrams for Tm3+ in YAG and GGG are obtained. The sets of crystal field parameters could be used to estimate a series of important spectral data for laser emission. A discussion of the spectral characteristics and the nature of Tm3+ multisite structure is also presented. The static spectral modifications in Cr3+ codoped with Tm3+ YAG or GGG crystals, due to mutual crystal field perturbations of the two dopants, are connected with Cr3+-Tm3+ pairs. The high- resolution data offer direct evidence that the Cr3+-Tm3+ energy transfer is governed for near neighbor Cr3+(a)-Tm3+(c) pairs by a short-range, most probably superexchange, interaction. The contribution of multipolar interactions should be considered only for more distant pairs, while at large distances, dipole-dipole interaction is dominant. The data suggest that the sensitization with Cr3+ determines not only an efficient way of transfer of excitation from the pumping sources to active ions but could produce changes of the spectral and temporal behavior of the latter.