Research into rare-earth ion doping of silica based optical fibres is described and a detailed study of the fabrication process reported. Dilute solutions of rare-earth ions are employed to incorporate dopants into preforms prepared by conventional methods. Alternative core glass compositions are also studied. Parameters affecting dopant incorporation, host composition and refractive index, as well as impurity concentration are investigated. Spectroscopic fluorescence and absorption measurements are presented and provide important information for device applications. In addition, fluorescence lifetime data allow the effect of dopant concentration and host composition to be analysed. Thus, a model describing the limits of doping levels is developed. The use of rare-earth ion doped fibre in several devices, both active and passive, is assessed, with special reference to the fabrication process. In particular, the effect of host glass on the laser characteristics of Tm3+ doped fibres and of the ratio of doping levels on the behaviour of Er3+/Yb3+ codoped fibre lasers is discussed. A practical temperature sensor based on fibre doped with low levels of rare-earth ions and a fibre filter requiring heavily doped fibre are demonstrated, with suggested design for improved performance. Finally, a novel radiation dosimeter, based on Nd3+ doped fibre, is described. Unusually, the response is found to be both rereadable and linear, even at elevated temperatures, and a model is proposed to describe the behaviour of this material. In summary, the fabrication, characterisation and applications of a wide range of rare-earth ion doped optical fibres have been investigated in detail.