In recent years, market definition has come under attack as an analytical tool of competition law. Scholars have increasingly questioned its usefulness and feasibility. That criticism comes into sharper relief in dynamic, innovation-driven markets, which do not correspond to the static markets on which the concept of the relevant market was modelled. This book explores that controversy from a comparative legal perspective, taking into account both EU competition and US antitrust law. It examines the manifold ways in which courts and competition authorities in the EU and US have factored...