Chemical and physical stresses cause various types of damage to stained glass windows; the internal and external environments, and the window structures themselves are all factors influencing deterioration processes. The paper suggests nondestructive in situ testing to obtain information, and maps on causes of deterioration. Tests should include microclimatic tests, strain gauge measurements, IR and UV photography and macrophotography, thermovision, spectrophotometry, and photoelastic methods. These are illustrated in work on the Cathedral windows at Orvieto.