Certain glasses made during the 17th and through the early 19th centuries have chemical compositions which are inherently unstable towards attack by moisture. This represents an alarming museological problem. Examples are known from England, the Continent, Scandinavia, China, and America. Historically, these glasses have been referred to as sick glasses. The sickness shows up in various forms, ranging from simple cloudy or dulled appearances to severe crizzling (or surface crazing), and occasionally even the repeated formation of moist, slippery surface films. No ideal treatments for crizzled objects seem yet to have been devised, but the use of proper storage conditions can at least forestall the occurrence of further damage until more satisfactory treatments become available. Relatively stable temperature and relative humidity combinations are probably to be preferred over cycling conditions such as can result from the periodic turning on and off of exhibition-case lights. Of particular concern are exhibition installations in which glass objects are spotlighted in order to present a dramatic effect. These conditions can cause localized heating and an attendant dehydration. -- AATA