IDFA (Identifier for Advertisers)

The primary device-level advertising identifier for Apple iOS devices, now gated behind user consent and significantly reduced in availability across the mobile ad ecosystem.

The Identifier for Advertisers (IDFA) is a unique, resettable device identifier assigned by Apple to iOS and iPadOS devices, designed to enable advertisers and app developers to track user behavior across apps and websites for targeted advertising and attribution purposes. Introduced in iOS 6 in 2012, IDFA became the dominant mobile advertising identifier on Apple's platform, allowing advertisers to measure campaign performance, build audience segments, and deliver personalized ads without exposing personally identifiable information directly. In April 2021, Apple fundamentally changed the IDFA landscape with the rollout of App Tracking Transparency (ATT) as part of iOS 14.5. Under ATT, apps must explicitly request user permission before accessing the IDFA, and industry data consistently shows opt-in rates ranging from roughly 25% to 45% globally, dramatically reducing the addressable pool of trackable users. This shift sent shockwaves through the mobile advertising ecosystem, significantly impacting companies like Meta, Snap, and countless mobile measurement partners (MMPs) that had relied on IDFA for attribution and retargeting. IDFA remains a critical — if constrained — signal in iOS advertising. When available (user opted in), it enables deterministic attribution and precise audience targeting. Its scarcity has accelerated adoption of alternative measurement approaches including SKAdNetwork (Apple's privacy-preserving attribution framework), probabilistic modeling, first-party data strategies, and contextual advertising. IDFA's evolution represents one of the most consequential privacy-driven disruptions in AdTech history, reshaping mobile measurement, identity resolution, and the competitive dynamics of the entire digital advertising industry.