The County Surveyor’s Society Bridges Group has produced with the assistance of TRL a Guide to the Repair and Strengthening of Arch Bridges. There are about forty thousand masonry arch bridges on the British road network, about forty percent of our total road bridge stock. The earliest still in existence are mediaeval and the amount and weight of traffic they are now called on to carry has increased enormously since they were built. It is important that they continue to perform their function because it would be neither practicable nor desirable to replace them. The cost would be enormous and many make a positive contribution to the landscape. This guide examines the problems common to arch bridges and the method of repair and strengthening which may be applied, together with estimates of their costs. It also describes the analysis methods which may be used to assess arch bridges and strengthening procedures, and the means of gathering the necessary data. The guide is designed to be a valuable source of information for young engineers whilst providing useful reference material for the more experienced.