Apple’s App Tracking Transparency framework, which required explicit user permission for cross-app tracking, reduced mobile advertising effectiveness in ways the industry understood theoretically in 2021 and has spent the subsequent years quantifying empirically. The quantification has not been favorable. CPMs for iOS advertising inventory dropped significantly in the period following ATT’s rollout. Attribution accuracy — the ability to connect an ad impression to a downstream app install or purchase — declined materially. The precision-targeted mobile advertising ecosystem that had been the dominant growth channel for consumer apps was not destroyed, but it was substantially impaired.