The supposed ubiquity of potential efficiencies is understood to justify permitting most horizontal mergers despite their tendency to raise prices. Yet efficiencies are said to be rarely decisive in actual merger decision-making. Moreover, the economic analysis of merger efficiencies lags far behind that of anticompetitive effects. This article addresses this analytical gap, drawing attention to the merger specificity of both efficiencies and anticompetitive effects, the teachings of neglected literature such as that on the theory of the firm, and the relevance of vertical efficiencies to horizontal mergers. The analysis is applied to economies of scale, economies of scope, and the sharing of assets between competitors. In addition, a focus on the long-run effects of merger policy shifts the debate on consumer versus total welfare (and regarding pass-through), alters the relevance of entry, and draws attention to endogenous asymmetries across firms and differences in the degree of competition across sectors of the economy. Finally, efficiencies are situated in a merger assessment framework, emphasizing how basic prescriptions of decision analysis conflict with official protocols for merger decision-making.