European merger regulation is a relatively new concept, established in 1990 to promote efficient competition in the Common Market. Since inception, merger regulation has undergone a significant transformation process that culminated in 2004 with the introduction of new guidelines for the assessment of horizontal mergers. The main focus of this book is to present a detailed analysis of current regulatory practice as related to the assessment of horizontal mergers in the EU. We provide an overview of merger control activity in the past two decades and discuss the main impact of the 2004 regulatory reform. We focus on changes in the assessment of horizontal mergers, present a comprehensive empirical analysis of key regulatory changes in European merger control and evaluate their real impact on the efficiency of merger regulation.