The suggestion that a NATO Advanced Study Institute would be an excellent forum for reviews and informed discussion on the broad subject of Nitrogen Ceramics, arose out of discussions with colleagues in the Department of Ceramics at the University of Leeds early in 1975. There was no doubt that such a meeting would be both very valuable and timely. Scientific and technological interest in the nitride ceramics and in silicon nitride in part­ icular had been growing steadily during the 20-year period following 1955. The intensive five-year programme initiated by the Advanced Research Projects Agency of the U. S. Department of Defence, on the development of a design capability in brittle materials for high temperature applications, had been based principally on silicon nitride and silicon carbide ceramics, and was due to reach the end of its first stage in the autumn of 1976. It was clear that by then a considerable volume of information covering many aspects of silicon nitride would be available for presentation or review. Coincidentally, the same five-year period had seen the discovery, and increasingly detailed investigation, of ceramic materials based on the AI-Si-N-O and similar systems. Besides being of great interest for their crystal chemistry and structural relationships, some of these materials could be assumed potentially to be of equal importance to the silicon nitride ceramics. More recently progress had also been made in the sintering of covalent materials, as demonstrated for the case of silicon carbide.