The Centre de recherches mathematiques (CRM) was created in 1968 by the Universite de Montreal to promote research in the mathematical sciences. It is now a national institute that hosts several groups and holds special theme years, summer schools, workshops, and a post-doctoral program. The focus of its scientific activities ranges from pure to applied mathematics and includes statistics, theoretical computer science, mathemat­ ical methods in biology and life sciences, and mathematical and theoretical physics. The CRM also promotes collaboration be­ tween mathematicians and industry. It is subsidized by the Nat­ ural Sciences and Engineering Research Council of Canada, the Fonds FCAR of the Province of Quebec, the Canadian Institute for Advanced Research, and has private endowments. Current activities, fellowships, and annual reports can be found on the CRM Web page at www. CRM. UMontreal. CA. The CRM Series in Mathematical Physics includes mono­ graphs, lecture notes, and proceedings based on research pur­ sued and on events held at the CRM. Yvan Saint-Aubin Montreal Preface This monograph is derived from a series of six lectures which were given at the Centre de recherches mathematiques (CRM) in Montreal, in March and June 2000, while the author was holder of the Aisenstadt Chair. Precis During the March 2000 meeting of the Workshop on Strings, Du­ ality, and Geometry in Montreal, Canada, three lectures were delivered on topics in fluid mechanics, while the author was holder of the Aisenstadt Chair.