We develop an incomplete-contracts model of endogenous market structure for a homogeneous-good industry. A large number of identical incentive-constrained producers each decide whether to stand alone as price-taking competitors, or to horizontally integrate with others by selling their assets to profit-motivated professional managers, who then Cournot compete in the product market. Despite the absence of significant technological non-convexities, the equilibrium market structure is typically an oligopoly: contracting imperfections can be a distinct source of market power. Unlike standard endogenous entry models, concentration may increase with the size of the market. We discuss implications for competition policy.