lowing other purchasers to combine it with other inputs and sell their finished products on a competitive basis, the input monopolist excludes certain purchasers either by internalizing production, and thereby becoming the sole source of finished goods, or by entering into strategic partnerships with certain purchasers and giving them preferential terms. The central concern of the essential facilities doctrine is thus vertical exclusion. 17 As such, the essential facilities doctrine is susceptible to - and, therefore, should reflect - the economic critique of vertical exclusion that has emerged over the past half-century (see Yoo 2002 for a review). In addition, the doctrine must take into account the emerging debate over the proper division of authority between antitrust courts and regulatory agencies in supervising access mandates. 1. The Economics of Vertical Exclusion As noted in Chapter 4, the conventional wisdom about vertical integration has undergone something of a sea change over the past half century. Antitrust schol- ars once generally accepted the notion that firms could use monopoly power in one level of production to reduce the competitiveness of vertically related lev- els of production (see, e.g., Kaysen and Turner 1959). Accordingly, the Supreme Court was quite receptive to vertical exclusion claims until the mid-1970s. In case after case, the Court struck down vertical mergers by firms controlling as little as 5 percent of the market, largely out of concern that vertical integration would reduce competition in the downstream market. 18 This hostility toward vertical exclusion became enshrined in the initial Merger Guidelines issued by the Justice Department in 1968, which disfavored vertical mergers involving firms controlling as little as 6 percent to 10 percent of their markets (U.S. Department of Justice 1968, § 12). The Supreme Court followed a similar pattern with respect to vertical contractual restraints, either holding them illegal per se¹9 or striking them down at such low 17 Werden (19