Upconversion phosphor materials are attracting considerable attention for their possible applications in solar cells with improved efficiency, nanomaterials for bio-imaging, lasers and novel display technologies. Upconversion materials, usually consisting of crystals doped with lanthanide ions, can convert low-energy incident radiation into higher energy emitted radiation. Several mechanisms are involved, including multiple photon absorption and energy transfers between dopants. Up to now, reported upconversion efficiencies have been relatively low, excitation thresholds quite high, and the investigated phosphors (generally fluorides) often presented poor chemical stability (hygroscopy), limiting their industrial applicability. In this dissertation, we investigate the upconversion luminescence characteristics of rareearth-doped RE2BaZnO5 (RE = Y, Gd) phosphors, for near-infrared to visible upconversion. Being oxides, these materials have good chemical, thermal and mechanical properties. A variety of dopants, including Yb3+, Er3+, Ho3+ and Tm3+, were embedded in the host lattice, resulting in bright red, green, blue and white light emissions under 980 nm excitation and at relatively low excitation powers. Upconversion efficiencies up to ~ 5.2%, 2.6%, 1.7% and 0.3% were respectively achieved in samples doped with Yb3+, Er3+ (green and red emission), Yb3+, Ho3+ (green emission), Yb3+, Tm3+ (blue and near-infrared emission) and Yb3+, Er3+, Tm3+ (white light emission). We believe that our green, red and white emitting systems are the most efficient upconverting samples reported to date for green, red and white light emission, which makes them serious candidates for many of the applications listed above. The upconversion mechanisms were determined for the first time by means of indepth steady-state and time-resolved spectroscopic investigations, including concentration and power dependence studies associated with temperature-dependent lifetime measurements.