This paper empirically analyzes the Federal Trade Commission's merger enforcement decisions, to supplement the 2004 release of the Horizontal Merger Investigation Data. The study provides insights into the review process for both multi- and single-market mergers. We present concentration-based models, customized to the relevant industry, for mergers with large numbers of overlaps. When more detailed data is available (for mergers with 3 or fewer overlaps), the analyses also focus on additional factors. We find evidence to suggest that, in addition to market structure, verified customer complaints and entry considerations also affect the enforcement decision. Finally, the study notes that the Commission's enforcement policy has been stable during the 1996 through 2003 time period.