Notwithstanding U.S. Supreme Court precedents hostile to mergers in general and to nonhorizontal mergers in particular (i.e., mergers and acquisitions that do not involve direct competitors), the prosecution of nonhorizontal mergers has virtually ceased. This monograph examines why this has happened and whether it reflects a desirable public policy. After a review of the historical development of nonhorizontal merger criteria, the monograph examines whether current antitrust policy should continue to be concerned about nonhorizontal mergers. Another issue addressed is whether nonhorizontal mergers create barriers to entry and raise rivals' costs in ways or with effects that should concern antitrust policymakers. Other concerns discussed are whether nonhorizontal mergers facilitate horizontal collusion and the evasion of rate regulation. The monograph concludes with a discussion of the principle that the vertical integration of two firms increases economic efficiency that may expand output and lower prices. 413 footnotes.