The main purpose of this book is to provide to a certain extent an answer to the question whether or not the European model will become the standard of antitrust law in the world, in light of the latest event, the Microsoft case. Comparing and evaluating EC Guidelines and U.S Guidelines in this book will offer some views of potential future outcomes concerning horizontal concentration. In the light of technological progress and globalized market, the biggest threat for the proper functioning of the market economy is indeed agreements and mergers with effect on horizontal concentration.