These volumes constitute the invited proceedings from the Ninth World Congress of the Econometric Society held on the campus of University College London on August 19–24, 2005.

As co-chairs of the Program Committee for the Congress, one of our most pleasant tasks was to select topics and authors for fifteen invited symposia – each organized around two papers.We chose topics for these invited papers that we thought were important, of current research interest, and where we could see a prospective long-run impact on the profession. All of the scholars that we first contacted agreed to contribute a paper. We encouraged them to write papers that would be of broad interest but would not necessarily be comprehensive literature surveys.

In the event, all symposia ran for two hours, during which the authors presented their papers and an invited discussant made comments on both of them. This book collects revised versions of the thirty papers presented in the fifteen invited symposia, as well as some of the comments by the discussants.

In all but one day of the congress, three invited symposia were run in parallel: one in economic theory, one in an applied field, and one in econometrics. The three volumes making up the book are organized by the same principle.

Volume I contains the papers on economic theory, broadly defined. In Chapter 1, “The Economics of Social Networks,” Matthew Jackson discusses a central field of sociological study, a major application of random graph theory, and an emerging area of study by economists, statistical physicists, and computer scientists.