The importance of the masonry arch in Britain's infrastructure has never been more significant especially at the present time under ever increasing axle loads. It is estimated that there are over forty thousand highway arch bridges in the UK alone of which the majority were constructed between the 17^th and 19^th centuries. As a result of recent European Community directives, a significant proportion of these are required to be reassessed to ascertain their load carrying capacity. The bridge assessment program in the UK is therefore urgent. Current methods of assessment are conservative and often result in unnecessary repair work. Improvements to these methods are now urgently required. This thesis therefore details a continuing study of the effects of the interaction of the soil and the structure on the overall strength of a multi-span arch bridge. The research involves testing of both small and large scale models in an attempt to quantify the effect of the pier on the overall failure load. Both instrumentation and image analysis are used to monitor the dispersal of stress throughout the various models. An assessment of the suitability of using finite element analysis to predict these stresses is also carried out. Finally, the results of a long term monitoring program of a new arch bridge using instrumentation installed during the construction is presented and the effects of seasonal temperature variations are investigated. A new technique for the assessment of arches is subsequently derived which is based on the regression analysis of the tests undertaken for this thesis. It is hoped that the results of this research can be quantified with the long term view of incorporating them in updated assessment methods.