The emergence of synthetic ceramics as a prominent class of materials with a unique combination of properties has been an important part of the materials-science scene over the past 20 years. These 'high-technology' ceramics have varied applications in areas utilizing their exceptional mechanical, thermal, optical, magnetic or electronic properties. A notable development of the 1970s was that of 'Si-based' ceramics (Si3N4' SiC and 'Sialons') as high-temperature engineering solids. More recently the zirconia-based ceramics have evolved as a class of material with significant improvements in fracture-toughness. In the 1980s we are on the threshold of development of ceramic-matrix composites with the promise of over­ coming major limitations in engineering design with 'brittle' ceramics and the development of novel properties unattainable with monolithic micro­ structures. Throughout this period there have been significant but less well-publicized developments in the field of glass-ceramics and glasses. It is the purpose of this publication to review selected topics within this important area of materials science. A key element in understanding the relation between properties and microstructure is a knowledge of atomic arrangement in ceramic phases. Recent developments in NMR and X-ray absorption spectroscopies have had considerable impact on studies of atomic co-ordination in glasses and crystalline ceramic materials and are reviewed in Chapters 1 and 2. Glass-ceramics are derived from the parent glasses by controlled crystal­ lization and have properties dictated, in part, by the efficiency of crystal nucleation within the glass volume.