The development of a fibre optic based system for the measurement of nitrate concentrations in water is reported. This system was designed to monitor nitrate concentration at levels appropriate to EC directives relating to drinking water quality. An experimental system was constructed to investigate direct UV transmission measurements at two wavelengths as a possible basis for a nitrate sensor. It was found that a dual wavelength measurement scheme was suitable since nitrates absorb strongly in the 210nm region but have negligible absorbance in the 275nm region. Limitations in the experimental system such as long data acquisition time and ouput drift meant that the experimental set up was not suitable as the basis for a portable nitrate detection system. A prototype optical fibre nitrate sensor for groundwater monitoring was constructed using a rotating filter wheel, lower attenuation fibre and a new electronic detection and signal processing system to overcome some of the limitations of the original experimental system. The perfomance of the prototype system was characterised in terms of range, sensitivity, resolution, accuracy and response time.The sensor exhibited excellent perfomance using short lengths of fibre but range and resolution performance were degraded somewhat when long lengths of fibre were used. However, the sensor was still capable of determining nitrate concentrations in the range of interest set by EC directives over long lengths of fibre.