This volume contains the proceedings of "PDE 2000", the international conference on partial differential equations held July 24 -28, 2000, in Clausthal. The confer­ ence took place during the EXPO 2000 and was sponsored by the Land Nieder­ sachsen, the Deutsche Forschungsgemeinschaft, the Bergstadt Clausthal-Zellerfeld and the Kreissparkasse Clausthal-Zellerfeld. This conference continues a series: Ludwigfelde 1976, Reinhardsbrunn 1985, Holz­ hau 1988, Breitenbrunn 1990, Lambrecht 1991 (proceedings in Operator Theory: Advances and Applications, Vol. 57, Birkhauser Verlag 1992), Potsdam 1992 and 1993, Holzhau 1994 (proceedings in Operator Theory: Advances and Applications, Vol. 78, Birkhauser Verlag 1995), Caputh 1995 and Potsdam 1996 (proceedings in Mathematical Research, Vol. 100, Akademie Verlag 1997). The intention of the organizers was to bring together specialists from different areas of modern analysis, mathematical physics and geometry, to discuss not only the recent progress in their own fields but also the interaction between these fields. The special topics of the conference were spectral and scattering theory, semiclas­ sical and asymptotic analysis, pseudodifferential operators and their relation to geometry, as well as partial differential operators and their connection to stochas­ tic analysis and to the theory of semigroups. The scientific advisory board of the conference in Clausthal consisted of M. Ben­ Artzi (Jerusalem), Chen Hua (Peking), M. Demuth (Clausthal), T. Ichinose (Kanazawa), 1. Rodino (Thrin), B.-W. Schulze (Potsdam) and J. Sjostrand (Paris).