Antitrust laws are concerned with controlling market power. In the course of history, the development of antitrust systems of market power control in the US and in the European Union (EU) has not followed a straight path. Legal practice, political ideology, and developments in economics have shaped an overcomplicated and undertheorized body of doctrine in relation to market power. Substantial ambiguity surrounds the definition, proof, and prevention of market power that deserves to be subject to antitrust law. By comparison and historical analysis, the present study seeks to shed some clarity on the issue.