Assembling novel data sets on online advertiser spending, performance, and revenue, we quantify the economic effects of Apple’s App Tracking Transparency (ATT) privacy policy on e-commerce firms. We find that conversion-optimized Meta advertisements, affected most by ATT, saw a 37% reduction in click-through rates after ATT. Although firms responded by shifting ad spending from Meta to the Google ecosystem, firms with higher baseline Meta dependence nevertheless experienced a substantial decline in firm-wide revenue relative to firms with lower baseline Meta dependence. We quantify these effects using a variety of methods, finding revenue decreases in the range between 8% and 40% relative to less exposed firms. These declines were primarily borne by smaller e-commerce firms, raising questions about the tradeoffs between consumer privacy and the ability of smaller e-commerce and direct-to-consumer firms to succeed in the product market. This paper was accepted by Jean-Pierre Dube, marketing. Funding: This work was supported by the LEC Program on Economics & Privacy, MSI Research Grant [4001820]. Supplemental Material: The online appendix and data files are available at https://doi.org/10.1287/mnsc.2024.06600 .