Abstract: Economic analysis of competition regulation is most developed in the domain of horizontal mergers, and modern agency guidelines reﬂect a substantial consensus on the appropriate template for merger assessment. Nevertheless, oﬃcial protocols are understood to rest on a problematic market deﬁnition exercise, to use HHIs and HHIs in ways that conﬂict with standard models, and more broadly to diverge with how economic analysis of proposed mergers should be and often is conducted. These gaps, unfortunately, are more consequential than is generally appreciated. Moreover, additional unrecognized errors and omissions are at least as important: analysis of eﬃciencies, which are thought to justify a permissive approach, fails to draw on the most relevant ﬁelds of economics; entry is often a misanalyzed afterthought; oﬃcial information collection and decision protocols violate basic tenets of decision analysis; and single-sector, partial equilibrium analysis is employed despite the presence of substantial distortions (many due to imperfect competition) in many sectors of the economy. This article elaborates these deﬁciencies, offers preliminary analysis of how they can best be addressed, and identiﬁes priorities for further research.