The wall paintings of Hardham Church, which date from c1100, are amongst the most important schemes of medieval painted decoration in England. Their iconography is also of international significance. Despite this significance, the wall paintings are in a fragile condition. Numerous attempts to repair them have largely addressed symptoms rather than causes, further compromising their condition. Consequently detailed investigations were undertaken by the Courtauld Institute of Art, University of London to identify the primary causes of the deterioration and the mechanisms by which they were activated. This was followed by targeted treatment testing and the development of an overall conservation strategy for this historic site. The methodology of the Hardham project offers an exemplary case study of diagnostic investigation and research. The publication will include a detailed record of the project, including the most recent innovative heating trials at the church. The work will be of interest to architects, surveyors, conservators, conservation scientists and others responsible for understanding the pathology of historic buildings.