The Federal Trade Commission and the Antitrust Division of the U.S. Department of Justice will soon publish new joint guidelines applicable to mergers that raise horizontal issues in the same market and to mergers that raise vertical issues in the same supply chain. These guidelines will reflect major changes in the perspectives of the two agencies with respect to the likelihood that a proposed merger or acquisition will lessen competition. They will also foreshadow decisions of the two agencies to challenge the legality of many proposed mergers and acquisitions that they would not have challenged in the past. When the agencies apply the new guidelines in an effort to block proposed mergers and acquisitions, courts will have an opportunit...