What it is
Every Android phone has a Google Advertising ID (sometimes called GAID or AAID) and every iPhone has an Identifier for Advertisers (IDFA). It looks like a long random string — something like 38400000-8cf0-11bd-b23e-10b96e40000d — and any app on your phone can ask the system for it.
It exists for exactly one purpose: to give advertisers a stable way to recognize the same device across different apps. Your weather app, your game, and your shopping app can each send that same ID to the same ad networks — and now those networks know all three belong to one person.
How one number becomes a profile
On its own, an ID is just a number. The profile forms through linkage:
- Cross-app linkage. Advertising SDKs embedded inside thousands of apps report the same ID, so your app list, usage patterns, and in-app behavior consolidate under one key. This is the mechanism behind Apple’s “Data Used to Track You” label — when an app declares it uses Identifiers for tracking, this is usually the identifier in question.
- Location attachment. Apps with location permission that also send the advertising ID effectively publish a movement history keyed to that ID. Location data sold by brokers has repeatedly been shown to be “pseudonymous” in name only — a device that sleeps at one address and works at another identifies its owner.
- Identity resolution. Data brokers link the advertising ID to emails, phone numbers, and home addresses gathered elsewhere (purchases, sign-ups, other datasets). At that point the “anonymous” ad profile is simply a file about you. The FTC’s data-broker studies documented brokers holding thousands of data points per consumer.
What changed recently — and what didn’t
Apple’s App Tracking Transparency (2021) made iPhone apps ask permission before reading the IDFA, and most people decline — a real improvement. Android lets you delete the ID entirely (it reads as a string of zeros afterward). What didn’t change: apps can still track you server-side using your account, your email (hashed), your IP address, and device fingerprinting. Resetting the ID trims the easiest linkage method; it does not end tracking. Honest expectations matter.
Reset or delete it (about a minute)
- Android: Settings → Privacy → Ads (on some phones: Settings → Google → Ads). Choose Delete advertising ID to remove it entirely, or Reset advertising ID to start a fresh one. While you’re there, review “Ad privacy” topics and turn off ad personalization.
- iPhone: Settings → Privacy & Security → Tracking → turn off Allow Apps to Request to Track (this zeroes the IDFA for apps). Then Settings → Privacy & Security → Apple Advertising → turn off Personalized Ads.
- Both: also visit your Google account’s My Ad Center (adssettings.google.com) — that controls profile-based ads tied to your account rather than your device.
Does resetting it break anything?
No. Apps work exactly the same; ads still appear, just less precisely aimed at you. There is no downside beyond seeing less “relevant” advertising — which is rather the point.
What this doesn’t fix
Advertising IDs are commercial tracking — companies profiling everyone. If your concern is a person tracking you, this page isn’t the fix: start with Is someone tracking my phone? and, if you may be in danger, Safety first.