This volume contains the Proceedings of the Sixth French-German Conference on Optimization, which took place in June 1991 at Lambrecht (PfaIz). About one hundred scientists, mainly from France and Germany, but also from other european countries, met at the Palatinate Conference Center in the heart of the beautiful palatinate national park to review and discuss recent developments in the field of optimization. More than sixty lectures were delivered, covering a large part of the theoretical and practical aspects of optimization. They all contributed to a stimulating scientific exchange during the meeting. This conference was the sixth in a series which started in 1980. Proceedings of the previous French-German Conferences on Optimization have been published as follows: First Conference (Oberwolfach 1980): Optimization and Optimal Control, edited by A. Auslender, W. Oettli and J. Stoer (Lecture Notes in Control and Information Sciences, 30). Springer Verlag, Berlin and Heidelber. e;, 1981 Second Conference (Confolant, 1981): Optimization, edited by J. -B. Hiriart-Urruty, W. Oettli, J. Stoer (Lecture Notes in Pure and Applied Mathematics, 86). Marcel Dekker, New York and Basel1983 Third Conference (Lnminy, 1984): Third Franco-German Conference in Optimization, edited by C. Lemarechal. Institut National de Recherche en Informatique et en Automatique, Rocquencourt, 1984 (ISBN 2-7261-0402-9) Fourth Conference (Irsee, 1986): Trends in Mathematical Optimization, edited by K. Hoffmann, J. -B. Hiriart-Urruty, C. Lemarechal, J. Zowe (International Series of Numerical Mathematics, 84). Birkhauser Verlag, Basel and Boston 1988 Fifth Conference (Varetz, 1988): Optimization, edited by S.