Why do mergers occur, what are their effects on social welfare and which are the best economic policies toward them? These three questions have been puzzling industrial economists since the end of the last century when the first great merger wave occurred in the United States. These issues have returned to the centre of the stage of the theoretical and empirical economic research since the mid-1980s as merger and acquisition activity became one of the most evident activities of firms in all industrialized countries. This study presents a look at the situation concerning competition policy both from the point of view of its basis hypotheses and different applications in the industrialized countries. The goal of the volume is to be useful not only to understand each country's situation but also as a point of departure for a thorough comparative research. In the globalization era, the objective of this research might be an exchange of experience in order to reach uniform positions on the most important subjects at a European and wider level.