This paper provides consistent understanding between the economic rationale of the ‘efficiency’ and ‘failing firm’ defenses and the conditions that the defenses require in the merger assessment process, focusing on two main concepts, ‘enhancing competition’ and ‘counterfactual’. This paper states that the economic rationale of the ‘efficiency’ defense rests on the effect of enhancing competition rather than on the improvement of efficiency itself. Regarding the ‘failing firm’ defense, the rationale is stated that competition would not deteriorate even when the merger proceeds compared to the counterfactual where it is prohibited. This understanding reflects not only recent international discussion but also the merger guidelines of advanced competition authorities. It also consistently explains the requirements of the defenses. Finally, this paper includes some comments on the Merger Review Guideline of Korea for its improvement based on various domestic merger cases.