Antitrust and competition laws are government regulations that seek to encourage competition by limiting the market power of firms. Some degree of monopolistic or market power has long been a feature of our economies and is most recognisable today through the activities of companies such as Google, Amazon, Meta, Microsoft and Apple. The concept of market power remains a central idea in fields such as industrial organization, the economics of regulation, competition law and competition policy, yet there is still much debate about how to define it and how to measure it. Antitrust and Competition Policy suggests a new approach for identifying market power and building on it sets out, for the first time, a sound, comprehensive economic foundation for competition law and policy. This framework sheds new light on a range of antitrust violations including the discernment of anti-competitive mergers, abusive practices and restrictive agreements.