The purpose of this report is to provide policy suggestions concerning welfare standards and efficiency defense in merger review. We start with an extensive review of relevant economic researches including theoretical and empirical studies of mergers and merger control. Next, we review approaches taken by competition agencies and courts in eight jurisdictions with respect to the principles and methods they apply in reviewing mergers that promise significant efficiencies as well as anti-competitive pressure. Then, we review the current legal practice in Korea and evaluate actual decisions made by the KFTC (Korea Fair Trade Commission) by gauging stock market reactions to the announcement of mergers. Finally, we provide conclusions suggesting the adoption of the Total Surplus Standard of the Canadian type and the need to consider efficiencies more favorably than before, provided that a safeguard correcting anti-competitive mergers ex post is set in place.