A study is made of both the manufacture and the characteristics of low-loss cladded optical fibres for use in the telecommunications network. A description is given of the evolution of the fibre drawing technology and of the development of several types of low-loss fibre. These include fibres having core materials composed of commercially available compound glasses, a specially-produced high-purity lead glass, an organic liquid and a vapour-deposited phosphosilicate glass. Attenuation figures range from 150dB/km to 1dB/km respectively. The propagation characteristics of the fibres are considered and it is shown that partial excitation of multimode step-index waveguides is both advantageous and practicable. Particular emphasis is placed on an investigation of the effect on pulse dispersion of preferential mode filtering and mode conversion. The thesis includes three new measurement techniques for the characterisation of optical fibres. The first is a method for the determination of the material dispersion in phosphosilicate glass over a wide wavelength range. Resulting from the measurement it is shown that a wavelength region exists in phosphosilicate fibres where the material dispersion is zero and the loss is small. It is suggested that operation at this wavelength would be preferable to that currently envisaged for optical communications. The second is a means for evaluating the waveguide parameters of a single-mode fibre. The method relies on observations of the far-field radiation pattern and permits the simultaneous measurement of both core diameter and refractive-index difference.