These notes are based on lectures delivered at Stanford University in January1 1994 and then repeated at MIT in the Spring semester. I am very grateful to the members of the Mathematics Department at Stanford, and in particular Ralph Cohen, for the invitation and hospitality. My especial thanks to those who attended the lectures and contributed in one way or another. I am particularly pleased to acknowledge the influence on my thinking of two of the members of the audience, Ralph Phillips and Joe Keller. Rafc Mazzco encouraged me to write up the lectures, provided me with his own notes and, as if that were not enough, made many helpful comments on the manuscript. I should also like to extend my thanks to Sang Chin, Daniel Grieser, Andrew Hassell, Mark Joshi, Olivier Lafittc, Eckhard Mcinrcnkcn, Edith Moocrs and Andras Vasy who attended the second hearing2 of the lectures at MIT and together made many useful remarks; Andras Vasy was particularly helpful in reading and correcting the notes as they dribbled out. I would also like to thank Tanya Christiansen and Gunthcr Uhlmann for their assistance and Lars Hormander, Georgi Vodev and Maciej Zworski for their comments on later versions of the manuscript.3
It is my hope that these notes may serve as an introduction to an active and growing area or research, although I fear they represent a rather steep learning curve.