Erica P. John Fund, Inc. (EPJ Fund) (plaintiff) brought suit against Halliburton Co. (Halliburton) (defendant), alleging that between June 3, 1999, and December 7, 2001, Halliburton made several misrepresentations regarding its revenue, a prospective merger, and potential liabilities. EPJ Fund filed a motion to certify as a class all investors that bought Halliburton common stock during this time period. Halliburton opposed the motion, arguing that the alleged misrepresentations had no impact on the price of Halliburton stock. Halliburton argued that this lack of price impact rebutted the presumption established in Basic Inc. v. Levinson, 485 U.S. 224 (1988), that investors rely on company misstatements in the buying and selling of the company’s stock. EPJ Fund argued that such rebuttal may be done only at the merits stage of a lawsuit, not at the class-certification stage. The court of appeals held that Halliburton may not rebut this presumption of reliance at the class-certification stage. Halliburton petitioned for certiorari, arguing that Basic should be overturned. The United States Supreme Court granted certiorari.