The Cathedral of Cologne, one of the most prominent monuments in Germany, is located in the extensively urbanized and industralized Lower Rhine valley. The metereological conditions result in a relatively mild and humid climate. In close vicinity to the Ruhr area, the Rheinschiene ranges also among the areas heavily affected by air pollution. The Cathedral's history implies a complex construction as well as a considerable variety (27 types) of stone materials. Four different stones, Schlaitdorfer Sandstone, Krensheimer Muschelkalk, Trachyte, and Obernkirchener Sandstone, are the major building materials. Except for the last one, all stones show damage, partly jeopardizing the safety of the monument. Extensive investigations on the correlation between air pollution and stone corrosion yielded clear evidence. The problem of damage analysis has been addressed by various mineralogical-petrographical, chemical, and petrophysical methods. Preservation tests relying on the crystallization method served as the basis for recent conservation campaigns. In case of unavoidable stone replacement, the Londorfer Basalt has proved an excellent substitute.