What is App Tracking Transparency (ATT)?

App Tracking Transparency (ATT) is Apple’s mechanism (since iOS 14.5) requiring apps to ask users for permission to track activity across other companies’ apps and websites. If the user denies (“Ask App Not to Track”), the app does not receive IDFA for third-party ads and analytics.

For mobile app media buying, ATT sharply reduced addressable iOS audience: lookalikes and retargeting weakened, modeled conversions and SKAdNetwork grew. For websites, Meta added Aggregated Event Measurement (AEM) with a cap on prioritized events per domain—a separate mechanism from ATT itself.

Arbitrage impact

Lower attribution and optimization precision on iOS.
Higher CPI/CPA on iOS vs. Android in many verticals.
Greater reliance on first-party data, email/phone match in CAPI, Android channels.

ATT prompt practices

Developers use a pre-prompt before the system dialog; opt-in rates vary by vertical (games higher, utilities lower). Media buyers purchase traffic into apps where the user already chose — the creative does not control the prompt directly.

Not the same as web cookies

ATT applies to native iOS apps. Web landings in Safari follow ITP and third-party cookie limits — a separate constraint set for browser arbitrage.