Panoramas et Synthèses is a new series of books edited by Société Mathématique de France. Each volume consists of a survey on a subject of active and present research. Panoramas et Synthèses is sold in the current year as separate issues or as a journal subscription. The AMS distributes Panoramas et Synthèses in the United States, Canada, and Mexico. Orders from other countries should be sent to the Société Mathématique de France (SMF), Maison de la SMF, case 916-Luminy-F-13288 Marseille cedex 09, France, or to Institut Henri Poincaré, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05, France. Members of SMF receive a 30% discount from list price. (ISSN 1272-3835) Softcover.