Respondents, Comcast cable television customers in the Philadelphia area, filed suit in the US District Court for the Eastern District of Pennsylvania against Comcast for violating antitrust laws in the provision of cable services. Respondents alleged that by acquiring various cable operations in the Philadelphia area in exchange for Comcast's cable operations in other parts of the country, known as clustering, Comcast amassed a 69.5% market share of the Philadelphia cable provider market. Respondents charged that the clustering activities violated Sections 1 and 2 of the Sherman Act by:

While Respondents submitted four theories of antitrust injury, the District Court found only one, that Comcast's conduct dissuaded potential competitors (or overbuilders) from entering the market, to be actionable. The District Court noted that to certify the class to be certified, damages from the antitrust impact must be measurable on a class-wide basis using common methods.

The District Court certified the class, reasoning that damages relating to the actionable antitrust injury could be determined class-wide using an expert-crafted model that compared actual cable prices in Philadelphia with hypothetical prices that would have been charged but for Comcast's alleged anticompetitive activities. That model, however, did not isolate damages resulting from each of the four theories of antitrust impact.

Comcast appealed the District Court's decision to the Third Circuit, arguing that because the model did not separately attribute damages to each of the four theories of antitrust injury, the class was improperly certified. The Third Circuit considered that argument to attack the merits of the certification methodology and did not consider it. The Third Circuit instead focused on whether Respondents were able to prove that damages could be determined without complicated calculations, which it found to be true and affirmed the decision. The court granted certiorari.

The court held that the Respondents' class was improperly certified under Rule 23(b)(3). The court limited the scope of its analysis to whether a district court may certify a class without considering whether the class has introduced evidence to show class-wide damages. The court stated that a model serving as evidence of damages must measure only the damages under the accepted theory of antitrust impact. The court reasoned that because the damages model used by the District Court and Circuit Court failed to measure only those damages attributable to the theory of antitrust impact considered by those courts, the Comcast subscribers could not be considered a single class. Therefore, the Third Circuit's ruling was reversed.

Writing for the dissent, Justices Ginsberg and Breyer argued that because Comcast did not object to the damages model at the certification stage, it forfeited any objection to its admission in later hearings and certiorari was improperly granted. However, even if Comcast had objected, the dissent found fault with the court's opinion because the plaintiffs can satisfy the predominance requirement even if damages are not provable in the aggregate. Therefore, the dissent argued that a class may be certified under Rule 23(b)(3) if common questions of liability predominate over unique questions of damages, and that the Third Circuit's ruling was correct.

The issue of whether the standards of admissibility of expert testimony apply in class certification proceedings was not addressed in the court's opinion. While both parties devoted much of their briefing to the question of admissibility, the court elected instead to focus on the issue of predominance and evaluate whether the plaintiffs had shown that this case was "susceptible to awarding damages on a class-wide basis."