As a basis for the conservation method currently being used for the windows in Erfurt Cathedral, the article provides an introduction to the medieval glass painting of the cathedral choir. This article briefly covers dates of manufacture, describes the inventory, deals with the history of glaziery and the state of the glass, Schwarzlot (flooding lead glass mixed with burnt copper) and lead, and finally explains the protective measures. Besides this, the longterm tests being undertaken are explained. The results will determine the preservation method to be used for the windows.