With the adoption of the 2020 Vertical Merger Guidelines, the U.S. antitrust agencies have updated their guidance on vertical mergers for the Twenty-First Century. Although economists have long recognized the procompetitive benefits most vertical mergers generate, the law has not always followed suit, and has sometimes condemned vertical mergers for making the merged firm more efficient. In this article, we attempt to catalogue the extensive list of efficiencies that vertical mergers can generate, trace the often halting efforts to incorporate these insights into the law, and propose a framework that courts and agencies can use to assess the likely competitive effects of vertical transactions. We draw heavily upon leading cases, particularly Baker Hughes and AT&T, with two refinements. First, consistent with the final Guidelines (but not the earlier draft) and the economic literature noting a symmetry between unilateral anticompetitive effects (raising rivals' costs) and procompetitive effects (the elimination of double marginalization), which we call the “unilateral effects tradeoff,” we argue a plaintiff alleging a raising rivals cost (RRC) theory of harm must also address EDM as part of its prima facie case. Second, if the plaintiff carries its prima facie burden, then the defendant should be able to argue, and courts and Agencies should seriously consider, the full range of procompetitive efficiencies, which we call a “holistic efficiency analysis.”