Based on a database of 368 merger cases scrutinized by the European Commission (EC) between 1990 and 2007, we evaluate the economic impact of the change in European merger legislation in 2004. We propose a general framework to assess merger policy effectiveness, focusing on four different policy dimensions: 1) predictability, 2) decision errors, 3) reversion of anti-competitive rents, and 4) deterrence. We compare the results before and after the reform, finding that the "more economic approach" resulted in improved ex-ante predictability of decisions and a reduction of the frequency of type I errors. Merger policy enforcement deters anti-competitive mergers without over-deterring pro-competitive transactions. Yet, the policy shift away from prohibitions, which are effective as a policy tool and as a deterrence mechanism, does not seem to be well grounded. (authors' abstract)