Using a combination of public and internal information, this paper compares and contrasts European Union (EU) and United States (US) merger policies. Common economic analysis leads both authorities to subject remarkably comparable portfolios of mergers to close scrutiny. Vertical mergers account for less than 10%, and potential competition matters for around 5%, of in-depth merger investigations in both jurisdictions, while purely conglomerate mergers are extremely rare or non-existent. The share of collusion investigations falls over time in both jurisdictions. However, the US relies on collusion theory more than three times as often as the EU, where over 80% of the horizontal cases address dominance. Across both regimes, roughly one eighth of all recent horizontal investigations have been analysed as non-dominance unilateral-effects cases. Only minor differences in the average probability of a merger being challenged are observed when controlling for market share. The 2004 EU reforms seem to be leading towards at least some convergence of enforcement policy.