In the decade since glass-ceramics first became mass-pro duced articles of commerce, they have become a popular subject for research and invention, as attested to by the 773 references cited in this book. Discovered almost accidentally during re search on photosensitive glasses, thermally crystallized glass ceramics have been distinguished by the rapid pace of their utili zation for distinctive new products. This promise has been recognized throughout the world, and original contributions have appeared from nearly every country having an ongoing glassmaking capability. Particularly numerous have been the publications and the ideas, scientific and technolOgi cal, issuing from the USSR. For several years, the annual All Union Conference on the Glassy State has been dominated by papers on catalyzed crystallization of glasses. With regard to new prod uct lines, we learn about slag-based sitaUs (glass-ceramics) and also about specialty items derived by radiation-assisted crystalli zation in glasses, photo-sitaUs. A. I. Berezhnoi has written a comprehensive review of the publications on this topic, which includes a balanced weighting to the contributions from the USSR and the USA, and also introduces advances from Britain, Czechoslovakia, Romania, Japan, and other centers of activity.