It is a widely held view that most internal church decoration, including plaster, was removed as part of the unprecedented campaign of restoration undertaken by the Church of England during the second half of the nineteenth century. The thesis seeks to test this view by quantifying the extent of medieval plaster surviving in Dorset churches, and setting this survival in an appropriate historical, technical and cultural context. Literature relating to church plaster on both a national and local level has been reviewed and correlated with the results of the survey and also used to explore cultural reasons for the destruction of medieval plaster. The survey has proved that there is a substantial survival of medieval plaster in Dorset churches. To date this survival has gone severely under-recorded, even in specialist literature. Perhaps the most significant finding of the survey has been the scale on which Victorian restorers have covered historic plaster with their own, rather than stripping the old and starting again from a bare substrate. Whilst this discovery has been a major success for the project, it has simultaneously highlighted the greatest weakness of the project. Since the survey is based solely on external visual examination, it has been unable to reliably estimate how much old plaster survives below Victorian overskim, only that there is peripheral evidence of its survival. Non-destructive testing systems that might overcome this problem are investigated and the results of trials reported. Techniques for improving the objectivity of visual survey are also reviewed. Survey data is analysed to determine if the probability of plaster survival can be predicted by factors such as location, date of restoration or architects involved.