Glossary
What is a device ID?
A device ID is a unique identifier assigned to a mobile device, used to track and attribute user activity across apps and advertising campaigns.
The main types
- [IDFA](/resources/glossary/identifier-for-advertisers-idfa) — Apple's advertising identifier, now gated behind ATT.
- [GAID](/resources/glossary/google-advertising-id-gaid) — Android's equivalent, resettable and deletable.
- [IDFV](/resources/glossary/identifier-for-vendor-idfv) — scoped to one developer's apps, no consent required.
- Hardware identifiers — IMEI, serial numbers and similar. Effectively inaccessible to apps on modern platforms.
- First-party identifiers — your own account ID, which is increasingly the only durable option.
Why they were central
Almost every measurement and targeting capability in mobile advertising reduced to one operation: recognising the same device twice. Attribution matched a click to an install. Frequency capping recognised a repeat viewer. Retargeting recognised a lapsed user. Audience building accumulated behaviour against an identifier.
Remove the ability to recognise the device and all four degrade simultaneously, which is exactly what happened.
What replaced them
Not another identifier — a different model. Aggregated reporting with privacy thresholds, contextual signals that need no identity, first-party login where the product earns one, and causal measurement that never depended on identifying anyone.
Conversational surfaces are worth noting here because they sidestep the problem rather than solving it. Targeting against what a user has just said needs no device identity at all, which is why identifier policy has essentially no effect on how AI advertising is targeted — see synthetic audience.
Common questions
Can I fingerprint a device instead?
Probabilistic fingerprinting to reconstruct an identifier is prohibited by both major platforms and carries regulatory exposure in most jurisdictions. It is not a workaround; it is a policy violation with a technical description.
What should I use instead of a device ID?
First-party account identity where the product justifies it, contextual signals where it does not, and aggregate causal methods for the measurement that identifiers used to support.
Where we write about device id
More in privacy and compliance
The identifiers, consent signals and rules that bound targeting.