This article is the first of two dealing with historical stained glass. The aim of this article is to provide an overview of the composition of historical glasses, to discuss the ensuing problems when attempting to restore medieval church windows, and to illustrate how scientific examination of glass surfaces can be used to select an appropriate cleaning method. The second articles, which is to appear in the next issue of Structure, will deal with the materialographic preparation and examination of historical glasses and modern copies produced for test purposes.