A primary benefit of mergers to the economy is their potential to generate significant efficiencies and thus enhance the merged firm's ability and incentive to compete, which may result in lower prices, improved quality, enhanced service, or new products. Efficiencies were originally claimed as a defense by the merging parties, but today “efficiency analysis” is a more integral part of the agencies’ investigation process. Both case law and the Merger Guidelines have evolved to give efficiencies a more active place in merger analysis. To be cognizable, a claimed efficiency must be verifiable, not attributable to reduced output or quality, merger-specific and greater than the transaction's substantial anticompetitive effects. All four of these requirements have evolved through time. This working paper attempts to list the case law in which the efficiency argument has prevailed and where it has failed.