The extent to which horizontal mergers deliver competitive benefits that offset any potential for competitive harm is a critical issue of antitrust enforcement. This Article evaluates economic analyses of merger efficiencies and concludes that a substantial body of work casts doubt on their presumptive existence and magnitude. That has two significant implications. First, the current methods used by the federal antitrust agencies to determine whether to investigate a horizontal merger likely rests on an overly-optimistic view of the existence of cognizable efficiencies, which we believe has the effect of justifying market-concentration thresholds that are likely too lax. Second, criticisms of the current treatment of efficiencies as too demanding—for example, that antitrust agencies and reviewing courts require too much of merging parties in demonstrating the existence of efficiencies—are misplaced, in part because they fail to recognize that full-blown merger investigations and subsequent litigation are focused on the mergers that are most likely to cause harm.