Visible upconversion fluorescence has been observed for laser excitation into the ⁴I₁₁/₂, ⁴I₉/₂ and ⁴F₉/₂ multiplets of Erᶾ⁺ ions in K₂YF₅ crystals. The predominant emission was from the ⁴S₃/₂ multiplet of Erᶾ⁺. For excitation into each multiplet the mechanism responsible for the visible upconversion emission has been shown to be two-step Sequential Absorption Upconversion (SAU). Significant blue and ultra-violet upconverted emission was also observed in each case. Various energy-transfer mechanisms are proposed to explain these higher energy emissions, which are thought to be subsequent to the initial SAU process. Optical absorption spectroscopy was performed in the region 1µm- 330nm on K₂YF₅: Erᶾ⁺. Absorption features were unambiguously assigned for transitions to the ⁴I₁₁/₂, ⁴I₉/₂, ⁴F₉/2, ⁴S₃/₂ , ²H₉/₂ , ²H₁₁/₂, ⁴F₇/₂, ⁴F₅/₂, ²H₉/₂, ⁴G₁₁/₂, and ²K₁₅/₂ multiplets of the Erᶾ⁺ ion. The assignments for the 4I₁₁/₂ to ⁴F₇/₂ multiplets confirmed the results of previous workers. Laser Selective Excitation (LSE) techniques have revealed evidence of a second crystalfield centre. The electronic transition energies of this centre differ only slightly from the primary centre indicating it represents a slight perturbation of the environment surrounding the Erᶾ⁺ ion, perhaps due to clustering. Various dynamical effects resulting from excitation into the ⁴I₁₁/₂ multiplet have also been observed. After inital excitation, fluorescence from the sample is seen to sharply rise, then slowly (τ ∼0.6s) attenuate to a steady level. A simultaneous reduction in both laser beam transmittance and ground-state population have also been observed. A trigonal C₃ᵥ symmetry Hamiltonian was fitted to the experimental crystal-field energy levels from the optical spectroscopy work The standard deviation of the fitted energy-level scheme (with respect to those experimentally observed) improved significantly on previous work.