Describes the nature of Romanesque, Gothic and Renaissance stained glass and explains why it is particularly susceptible to damage from modern air pollution. Describes restoration techniques used in the past, many of which caused further deterioration of the glass, and current conservation techniques. Observes that a controlled environment for stained glass is the ideal. See also AATA (6)-F4-49 and 20-734 for previous discussions of medieval stained glass painting technique and restoration. -- AATA