European Community directives now insist upon the imposition of 11.5t axle weights for the assessment of highway bridges and structures. This need for heavier loads arises from the Community wide harmonisation of transport policy. Its successful implementation requires the urgent assessment of our bridge stock of some 75000 masonry arches. The analysis of arch bridges has long lacked an accurate method of assessing the loads transmitted to the arch ring by the surrounding soil. This thesis proposes pressure distributions suitable for use in the analysis of arch bridges. It examines, by way of instrumented small scale and in-situ tests, the soil-structure interaction effects arising from the backfill material. Observations of zones of soil displacement around a loaded arch are made in order to better describe the interactive effects. A finite element analysis of the instrumented tests was done and a parametric study was used to assess the effects of various material properties upon the system's behaviour. The inclusion of the interactive effects observed, and modelled, intends to lead to cost savings in the arch bridge assessment programme by reducing the conservatism inherent in the most common assessment methods. Design curves incorporating soil-structure interaction effects are presented where significant capacity increases can be seen compared with analyses ignoring the effects.