This thesis describes the novel multicomponent glasses as the basis for fibre optic device and system applications. It is shown that multicomponent glasses are expected to greatly enhance the functionality of the fibre-optical devices and systems by compensating for the limitations of current silica-glass based fibre-devices. Fabrication and characterization of various multicomponent glasses and fibres suitable for specific applications have been investigated. The fabrication of rare-earth-doped multicomponent glasses based on commercial optical-glasses has been tackled by developing a novel doping technique. An intermediate maximum phonon-energy lead-germanate glass suitable for fibre optic applications has been achieved by optimizing the glass composition via the Levin-Block concept. Non-hygroscopic chlorotellurite glass. 60TeO2. 40ZnCl2, and its modifications for fibre structure have been prepared, their intrinsic high thermal-stabilities characterized and intrinsic optical transparency in the mid-IR evaluated. Spectroscopic and structural characterization of rare-earth-doped multi-component glasses and fibres have also been conducted. Radiative and non-radiative processes for rare-earth transitions in glasses were studied with a view to finding materials in which rare-earth transitions are more radiative and less non-radiative. A "crystal-chemistry" model is proposed to evaluate the rare-earth local structure in glass, based on the Modified-Random-Network (MRN) theory in glass structure. The rare-earth spectral behaviour in glasses has been well described and predicted by this model. Using the rare-earth-doped novel multicomponent glass-waveguides has led to a number of fibre and planar-waveguide laser devices having been demonstrated. This includes the most efficient and a high-power Nd3+- fibre laser ever reported at 1.06µm in oxide glasses, the first demonstration of Nd3+- and Er3+- doped monomode planar-waveguide lasers at 1.06µm and 1.54µm and a new class of fibre laser based on rare-earth-doped lead-germanate glass.