App Tracking Transparency (ATT)

App Tracking Transparency, or ATT, is a privacy framework Apple introduced with iOS 14.5 in April 2021. It requires apps to ask users for permission before tracking their activity across apps and websites owned by other companies, including access to the device's advertising identifier, known as the IDFA. If a user declines, the app must not track them in that way.

Why it mattered for advertisers

Before ATT, ad platforms could more easily connect what a person did after clicking an ad, such as buying on a website, to the ad they saw in an app. Once many iOS users opted out, that connection became harder for platforms such as Meta, with several knock-on effects:

• Reported conversions became less complete, especially for iOS traffic.
• Meta introduced Aggregated Event Measurement and changed its default attribution window, removing 28-day windows.
• Retargeting pools and lookalike sources based on website activity could shrink.
• Optimisation had less signal to work with.

How the industry adapted

• Server-side tracking: more advertisers adopted the Conversions API alongside the Meta Pixel, while still respecting consent.
• Blended metrics: many brands put more weight on MER and store data rather than platform-reported ROAS alone.
• Incrementality testing: measuring real impact rather than relying on attribution.
• Broader targeting and creative focus: with less precise data, broad targeting and creative quality became more important.

Platform measurement continues to evolve, so check current documentation for how ATT affects reporting at the time of writing.