Caroline Behrend and 2 million others who were similarly situated (plaintiffs) filed a class-action suit in federal district court against cable-television provider Comcast Corporation (Comcast) and its subsidiaries (defendants), seeking damages for violations of federal antitrust laws. The plaintiffs claimed that Comcast had attempted to monopolize the cable market in Philadelphia, Pennsylvania, through a series of swap transactions with cable providers in other markets that resulted in the elimination of competition and increased prices. The plaintiffs filed a motion for class certification under Federal Rule of Civil Procedure (FRCP) 23(b)(3) and proposed four different theories of antitrust impact. An expert for the plaintiffs, Dr. James McClave, compared actual cable prices in the Philadelphia market with hypothetical prices that would have prevailed but for Comcast’s conduct, and fashioned damages of over $875 million for the class. In granting the motion for class certification, the district court accepted only one of the plaintiffs’ theories, namely that Comcast’s conduct reduced the level of competition from companies that built competing cable networks in areas where an existing cable company operated. This was known as the overbuilder theory. Although the model did not indicate how the overbuilder theory affected the price of cable-television services in the area, the district court concluded that damages from decreased competition could be calculated on a classwide basis. The defendants appealed. The court of appeals affirmed, concluding that Dr. McClave was not required to correlate damages to likely antitrust impact based on Comcast’s conduct at the class-certification stage, because this would involve an analysis of the merits of the case. The United States Supreme Court granted certiorari to review.