In his comprehensive study On the Relevance of Market Power, Professor Louis Kaplow sets forth “a ground-up exploration of the proper role of market power” in competition law. Kaplow’s thorough and insightful analysis of what he calls the three “channels of potential relevance” sheds important new light on how understanding market power can aid the assessment of conduct that is alleged to be anti-competitive. It should be required reading for anyone engaged in the enforcement of competition law. But Kaplow focuses broadly on “competition law,” which encompasses both U.S. antitrust law and antitrust-type laws in other countries. His analysis is thus very abstract and does not appreciate the critical role that market power plays in U.S. antitrust law as a separate element of the antitrust offense.