In Comcast Corp. v. Behrend, No. 11-864 (U.S. March 27, 2013), the U.S. Supreme Court heightened the requirements for plaintiffs seeking class certification. The ruling has significant implications for antitrust class actions in particular and for class actions in general. In a 5-4 decision building on its 2011 opinion in Wal-Mart v. Dukes (see June 22, 2011 Sidley Update), the Court reversed the certification of a class because of plaintiffs’ inability to demonstrate a way to compute classwide damages in a manner consistent with the theories of liability in the case. This decision will require district courts to pay close attention to plaintiffs’ theories of classwide damages before deciding whether to certify a class.