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The article discusses economic analysis at the class certification stage of Exchange Act securities class actions, focusing on three key issues: market efficiency, price impact, and classwide damages methodology. The US Supreme Court's Basic decision established the "fraud on the market" theory, which holds that reliance can be presumed if the security traded in an efficient market. To invoke this presumption, plaintiffs must show that the security traded in an efficient market. Defendants can challenge class certification by rebutting the Basic presumption or by providing direct evidence that alleged misrepresentations did not impact the security's price. The article also explores factors considered in assessing market efficiency, including structural factors and empirical evidence of a cause-and-effect relationship between new information and security price changes. Event study analysis is a key tool in assessing market efficiency, and economists may use it to demonstrate that a security's price reacted quickly to unexpected, value-relevant company news.