This is the first of three volumes containing edited versions of papers and a commentary presented at invited symposium sessions of the Ninth World Congress of the Econometric Society in 2005. Written by leading specialists in their fields, these volumes provide a unique survey of progress in the discipline. Contributors ix Introduction by the Editors xi The Economics of Social Networks 1(56) Matthew O. Jackson Multi-Contracting Mechanism Design 57(45) David Martimort Allocative and Informational Externalities in Auctions and Related Mechanisms 102(34) Philippe Jehiel Benny Moldovanu The Economics of Relationships 136(50) Larry Samuelson Information in Mechanism Design 186(36) Dirk Bergemann Juuso Valimaki Communication in Economic Mechanisms 222(47) Ilya Segal Advances in Dynamic Optimal Taxation 269(29) Narayana R. Kocherlakota Quantitative Macroeconomic Models with Heterogeneous Agents 298(43) Per Krusell Anthony A. Smith, Jr. Modeling Inefficient Institutions 341(40) Daron Acemoglu Whither Political Economy? Theories, Facts and Issues 381(41) Antonio Merlo Comments on Acemoglu and Merlo 422(9) Timothy Besley Index 431