Optical absorption and laser selective excitation (l.s.e.) spectroscopy have been used to study Er³⁺ ions in the CsCdBr₃ and hydrogenated CaF₂ and SrF₂ crystalline lattices. Sixteen hydrogenic charge compensation sites for both the H- and D- isotopes were found in CaF₂:0.05%Er³⁺, and nine new D- ion sites in SrF₂ :0.05%Er3³⁺. The relative occurrence of these sites can be controlled by variation of the duration of hydrogenation. For SrF₂:Er³⁺, this results in a change of the principal D- ion charge compensation arrangement. Classification of local-mode vibronic fluorescence transitions and consideration of isotope shifts of electronic transitions enabled the assignment of model configurations to several sites. Most hydrogenic sites were classified into one of two distinct families. A new effect, reversible polarised bleaching was observed, whereby hydrogenic ion migration in Er³⁺ ion site configurations can be controlled by l.s.e. with polarised light in oriented crystals. One hydrogenic site was found to consist of two photo-induced interconvertible components. Five new fluoride ion sites in CaF₂:Er³⁺ and nine in SrF₂:Er³⁺ were also determined. For <100> and <111> oriented crystals, the fluorescence spectra of several sites exhibited well defined polarisation which may be used to determine the Er³⁺ ion symmetry. Trigonal symmetry of the F- B site of CaF₂:Er³⁺ was confirmed by this means. Crystal field analysis of tetragonal and trigonal symmetry sites is also reported. An infra-red study revealed local mode lines of seven new sites in hydrogenated CaF₂:Er³⁺ and enabled correlation of some sites to those observed in the optical spectra. Reduction of Er³⁺ ions by the short range beta decay of tritium in the lattice was identified by local mode absorption. Erbium ions in CsCdBr₃ exhibited efficient upconverted fluorescence consistent with the formation of a dominant dimer site. Detailed spectroscopy resulted in the assignment of energy levels from twelve groups of transitions. Several of these transitions, studied by absorption and l.s.e., were found to consist of two components.