This chapter discusses the adequacy of the single entity doctrine in complex horizontal relationships, by which competitors are tied both by a market and a corporate relation. One critical implication is that agreements between competitors and their (lawfully formed) joint venture may be exempted from further scrutiny. In such cases, the single entity doctrine, if applied consistently with the principles of the theory of the firm, adequately captures anticompetitive effects of corporate arrangements. The competitive effects of other arrangements, however, may fall short of antitrust scrutiny. In the EU and in the US, concern was raised over possible anticompetitive effects of financial ownership links which may be unchallenged. Interlocking directorates, created by directors sitting on the board of several companies, can also create uncontrolled anticompetitive risks. In the EU an enforcement gap exists because structural links do not fit into the firm/market paradigm underlying the application of Articles 101 and 102 TFEU and Merger Control. In the US, Sections 7 and 8 of the Clayton Act have substantive reach over partial acquisitions and interlocking directorates irrespective of such paradigm. Capturing anticompetitive effects of structural links may require adjustment of the substantive reach of competition law vis-à-vis, and greater insights into, corporate governance.