Auhor's comments: Although I have the feeling of having left unfinished almost every mathematical project undertaken, the study of endoscopy and the stabilized trace formula was, in this respect, one of the most unsatisfactory of all. It went on for a very long time without reaching any very cogent conclusions. This now seems with hindsight to have been inevitable. The efforts of a number of excellent mathematicians make it clear that the problems to be solved, many of which remain outstanding, were much more difficult than I appreciated. In particular, the fundamental lemma which is introduced in these notes, is a precise and purely combinatorial statement that I thought must therefore of necessity yield to a straightforward analysis. This has turned out differently than I foresaw.
Without the kind invitation of Marie-France Vignéras to deliver lectures at the École normale supérieure de jeunes filles, I would never have attempted to communicate the inchoate results at my disposition and I would have continued, no doubt unsuccessfully, to struggle with problems, both local and global, that were beyond me. The lectures were an occasion to clarify and organize the few ideas that I had, and have served as a stimulus to other, more competent, investigators.