The development of a fibre optic oxygen sensor based on the fluorescence quenching of ruthenium complexes entrapped in a sol-gel-derived microporous silica film is presented. The sensor is based on evanescent wave excitation of the sol-gel film which is coated on a declad portion of optical fibre. Theoretical considerations concerning the efficiency of evanescent wave excitation of fluorescence and its collection are discussed in detail. Experimental measurements which investigate these predictions are presented. The principles of fluorescence quenching are outlined and sensing based on measuring fluorescence intensity or fluorescence decay time are described. In particular the advantages of the phase fluorimetric method of decay time monitoring are highlighted. The ruthenium complex [Ru“-tris(4t7-diphenyl-1,10-phenantroline)] was chosen for this work and was immobilised in a microporous glass, produced by the low temperature sol-gel process. The advantages of using this method of reagent capture are explained and the method of sensor fabrication described. A number of experimental systems have been employed. The progress from an air-cooled argon-ion laser/PMT characterisation system to a compact LED/photodiode system is detailed. The oxygen sensor was found to exhibit fast response times of less than 5 seconds, high sensitivity to oxygen and good repeatability. Sensor response, including dependence on oxygen concentration, temperature and humidity are discussed and the viability of manufacturing this type of sensor as an industrial product is examined. In addition, related work leading to the development of fluorescence-based evanescent-wave immunosensor is presented. Lactate dehydrogenase, a clinically important diagnostic marker enzyme, is detected in concentrations as low as 30ng/ml.