This essay provides a perspective on the role of antitrust law in protecting and fostering competition in the digital economy, with particular attention to online platforms. It highlights the danger of anticompetitive exclusionary conduct by dominant online platforms and describes ways that antitrust law can challenge and deter such conduct. The essay also identifies a number of difficulties that U.S. courts and enforcers face in challenging harmful exclusionary conduct by dominant platforms, and discusses some ways that regulation can supplement antitrust law in fostering competition.