(Reprint of Saint-Flour Lecture in 2000, first published in S. Albeverio, W. Schachermayer, M. Talagrand: Lectures in Probability and Statistics.
(Nachdruck der Saint-Flour Lecture 2000, erstveröffentlicht in S. Albeverio; W. Schachermayer; M. Talagrand: Lectures in Probability and Statistics.