These contributions discuss a number of important developments over the past decade in a newly established and important field of economics that have led to notable changes in views on governmental competition policies. They focus on the nature and role of competition and other determinants of market structures, such as numbers of firms and barriers to entry; other factors which determine the effective degree of competition in the market; the influence of major firms (especially when these pursue objectives other than profit maximization); and decentralization and coordination under control relationships other than markets and hierarchies. The contributors are Joseph E. Stiglitz, G. C. Archibald, B. C. Eaton, R. G. Lipsey, David Enaoua, Paul Geroski, Alexis Jacquemin, Richard J. Gilbert, Reinhard Selten, Oliver E. Williamson, Jerry R. Green, G. Frank Mathewson, R. A. Winter, C. d'Aspremont, J. Jaskold Gabszewicz, Steven Salop, Branko Horvat, Z. Roman, W. J. Baumol, J. C. Panzar, R. D. Willig, Richard Schmalensee, Richard Nelson, Michael Scence, and Partha Dasgupta. Joseph E. Stiglitz is Professor of Economics at Princeton University. G. Frank Mathewson is Professor of Economics at the University of Toronto.