Apple is turning the privacy settings of its mobile ecosystem upside down. When it releases its app tracking transparency (ATT) framework with iOS 14.5 on April 26, it will shut off a stream of data that app developers, measurement companies, and advertisers have used to link users’ behavior across apps and mobile websites — a move that could reshape the digital advertising industry. With the update, the “identifier for advertisers” (IDFA), which has been activated by default on Apple devices and provides access to user-level data to app publishers, will be switched off and users will need to grant apps explicit permission to access it. With in-app prompts asking users, “Allow [app name] to track your activity across other companies’ apps and websites?” opt-in rates will likely be low.