Several schemes of laser excitation of mid-IR (4-6 μm) transitions for the Er 3+ , Dy 3+ , Pr 3+ , Nd 3+ , and Ce 3+ ions were considered. Among them is cooperative excitation of the 4 I 9/2 initial laser level of Er 3+ in concentrated samples under 1.54 μm of erbium glass laser and two step excitation by the same laser as well as direct excitation into the 4 I 9/2 initial laser level by Ti:Al 2 O 3 laser. For Dy 3+ different excitation schemes including excitation into the 6 H 9/2 ; 6 F 11/2 manifold lying higher than the 6 H 11/2 initial laser level by 1.3 μm ofYAG:Nd or oxygen iodine laser, or by 1.064 μm of YAG:Nd laser into the 6 H 7/2 ; 6 F 9/2 higher lying manifold, or sensitization of the next higher lying 6 H 5/2 manifold by Yb 3+ are considered. For Pr 3+ cooperative excitation by 2 μm of thulium laser into the 3 H 6 manifold as well as direct excitation of 3 F 3 initial laser level by 1.54 μm of erbium glass laser and erbium sensitization of the same level are under consideration. For Nd 3+ direct excitation of 4 I 15/2 initial laser level by 1.68 μm of YAlO 3 :Er laser as well as erbium sensitization of the same level pumped by 1.54 μm of erbium glass laser is considered. Recommendations for selection of the type of active solid state matrix for 4-5-μm laser oscillation for different rare-earth ions are proposed. A 4-5-μm fluorescence spectra of Dy 3+ in the new La; Ga; Ge sulfide glass and the KPb 2 Cl 5 crystal, as well 4-6-μm fluorescence spectra of Dy 3+ , Pr 3+ , and Nd 3+ in the KPb 2 Cl 5 crystal were measured at room temperature. Emission and absorption characteristics of the 6 H 9/2 ; 6 F 11/2 , 6 H 11/2 , and 6 H 13/2 levels suitable for mid-IR laser generation scheme in the new La; Ga; Ge Dy 3+ doped sulfide glasses were calculated from the absorption spectra. The fluorescence kinetics decay for the 6 H 9/2 ; 6 F 11/2 , 6 H 11/2 , and 6 H 13/2 manifolds of Dy 3+ in the new sulfide glass was measured and compared with that for the other known sulfide and chloride solid state laser matrixes doped with dysprosium. The excitation scheme for sensitization of Ce 3+ 4-6-μm transition by erbium using Ti:sapphire or diode laser pumping into the 4 I 9/2 manifold is realized in fluoride crystal matrix. The fluorescence spectrum of Ce 3+ in the La 1-x Ce x F 3 solid solutions co-doped with erbium for Er → Ce fluorescence sensitization was measured in the 3.5-5.5-μm spectral region at room temperature. Fluorescence kinetics decay of the 4 I 9/2 initial 4.6 μm laser level of erbium in the fluoride crystals with fluorite structure (CaF 2 , SrF 2 , BaF 2 , CdF 2 , and PbF 2 ) and in the LaF 3 crystal with hexagonal structure with short phonon spectra was measured at room temperature and 77 K. It was found that in the row of fluoride crystals doped with erbium the governing factor influenced multiphonon relaxation (MR) rates is the number of phonons n, which is determined by the maximum frequency of longitudinal optical phonons (ω max ). A decrease of the maximum phonon frequency ω max raises the number of phonons n. This decreases the MR rate and increases the fluorescence lifetimes for PbF 2 and BaF 2 . The latter fact could be favorable in attaining laser generation in the 4-5-μm spectral region where multiphonon relaxation usually dominates. Influence of the unit cell dimension of crystal lattice (rare-earth ion to ligand distance R 0 ) on the MR rate is discussed.