We review the literature analyzing how boundaries of firms (the way suppliers are integrated or contracts that are used among stakeholders) are influenced by the degree of product market competition as well as the industrial organization literature on endogenous market structures ( (papers by K. Schmidt, X. Vives, R. Kranton and D.F. Minnehart, M. Raith, J. Sutton). We then show how organizational considerations, in particular contracting incompleteness, provide novel mechanisms for explaining the emergence of different market structures, hence the degree of market competition (paper by P. Legros, A. Newman, U. Zsolt).