Panoramas et Synthèses is a series of books edited by Société Mathématique de France published in English or in French. The main goal of this series is to provide a synthetic viewpoint on a given mathematical topic. Contributions sometimes come from meetings organized in the same spirit, but the origin can be a more personal initiative. The authors attempt to focus on the main ideas, intuitions, and guidelines of a mathematical field and to give a sketch of the most recent developments of the subject. 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.