I should emphasize that what I am going to talk about is really the result of a very brief program which was funded at Harwell by David Godfrey of AMTE, who is probably well-known to ceramic practitioners here. We were very glad to have this contact because it gave us access to the TTCP specimens which were prepared in this country; specimens, in particular, of silicon nitride tiles containing seeded defects, which many of you have studied by various techniques.