The aim of this monograph is to give an overview of various classes of in'ni- dimensional Lie groups and their applications, mostly in Hamiltonian - chanics, ?uid dynamics, integrable systems, and complex geometry. We have chosen to present the unifying ideas of the theory by concentrating on speci'c typesandexamplesofin'nite-dimensionalLiegroups. Ofcourse, theselection of the topics is largely in'uenced by the taste of the authors, but we hope thatthisselectioniswideenoughtodescribevariousphenomenaarisinginthe...