Thou shalt not grant or deny class certification based on an analysis of the probability of success on the merits of the plaintiffs’ claim. That fundamental principle of procedural jurisprudence was announced over 25 years ago in Eisen v. Carlisle & Jacquelin and remains bedrock law today. But the principle has had increasingly negative consequences, has helped lead to an undue narrowing of substantive antitrust rules, and is no longer supported by the reasons originally given for it. This Article, therefore, suggests that Eisen has outlived its usefulness and should be overruled.