Much of this book was written during a sabbatical visit by J. C. H. S. to the Max Planck Institute in Stuttgart during 1991. We are therefore grateful to Professors M. Ruhle and A. Seeger for acting as hosts during this time, and to the Alexander von Humbolt Foundation for the Senior Scientist Award which made this visit possible. The Ph. D. work of one of us (J. M. Z. ) has also provided much of the background for the book, together with our recent papers with various collaborators. Of these, perhaps the most important stimulus to our work on convergent-beam electron diffraction resulted from a visit to the National Science Foundation's Electron Microscopy Facility at Arizona State University by Professor R. H(lJier in 1988, and from a return visit to Trondheim by J. C. H. S. in 1990. We are therefore particularly grateful to Professor H(lJier and his students and co-workers for their encouragement and collaboration. At ASU, we owe a particular debt of gratitude to Professor M. O'Keeffe for his encouragement. The depth of his under- standing of crystal structures and his role as passionate skeptic have frequently been invaluable. Professor John Cowley has also been an invaluable sounding board for ideas, and was responsible for much of the experimental and theoretical work on coherent nanodiffraction. The sections on this topic derive mainly from collaborations by J. C. H. S. with him in the seventies.