What Is Ad Tracking, and How Does It Follow You

In short: Ad tracking is the practice of collecting data about what you do online — and increasingly across your phone, tablet and computer — to build a profile used for targeted advertising. The Federal Trade Commission has described this cross-device version of tracking as something that "often takes place without consumers' knowledge," and notes people have limited choices to control it.

If you've ever searched for running shoes on your laptop and then seen an ad for the same shoes on your phone an hour later, you've felt ad tracking at work. It's the broad category that sits above things this site has already covered one at a time — tracking cookies on a single browser, the advertising ID baked into your phone, Apple's prompt asking an app not to track you. Ad tracking is the whole system those pieces are part of: the methods, the identifiers, and the business of linking your activity across websites, apps and devices into one profile that advertisers can target.

What exactly is ad tracking and why does it happen?

Ad tracking is the collection of data about a person's behavior — pages visited, apps opened, searches made, locations passed through — so that data can be used to decide which ad to show that person next, or to measure whether an ad worked. The Federal Trade Commission's staff report on the subject describes a specific form of this called cross-device tracking, which it defines as the practice that "associates multiple devices with the same consumer and links a consumer's activity across her devices (e.g., smartphones, tablets, personal computers, and other connected devices)," according to the FTC's 2017 press release on the report.

From an advertiser's side, this is simply targeting: knowing enough about a person to show them a relevant ad, and enough about what happened after to tell whether it worked. From your side, it means a company you've never directly dealt with may hold a profile of your browsing and app activity, built without you ever filling out a form or clicking "accept" on anything resembling a survey.

How do advertisers follow you across different websites and devices?

They do it by linking separate pieces of activity — a visit to one site, a purchase on another app, a search on a third device — back to the same person. The FTC's report found this frequently happens invisibly to the consumer involved; its own summary states plainly that cross-device tracking "often takes place without consumers' knowledge," and that when it works, it can create larger stores of sensitive data that need protecting, per the FTC's cross-device tracking report.

Because this kind of linking is largely invisible and hard for an individual to audit, the FTC staff who wrote the report didn't just describe the problem — they laid out expectations for companies doing it. The report "encourages entities" engaged in cross-device tracking to truthfully disclose the practice to consumers, offer people choices about how their activity gets tracked, get affirmative consent before tracking sensitive topics or precise location, and keep the data secure, according to the FTC report. Those four points — disclose, offer choice, get consent for sensitive data, secure it — are a useful checklist for judging whether any company's tracking practices meet a baseline standard.

What tools do trackers use — cookies and device IDs?

This site has already walked through two of the main tools individually. A tracking cookie is a small file a website leaves in your browser so it (or an ad partner) can recognize you on a later visit or on another site; if you want the mechanics of removing them, the guide on how to get rid of tracking cookies covers that. A device-level identifier works differently: it's a code tied to your phone or tablet itself rather than to a browser session, and it's what lets an app-based ad network recognize you across different apps. The explainer on what advertising IDs do covers how that number works on a phone.

Ad tracking is the umbrella that holds both of those, plus the practice of linking them together. A cookie on your laptop browser, a device ID on your phone, and whatever account you're logged into on a streaming box can all point back to the same advertising profile once a company has a way to connect them — which is exactly the linking the FTC's cross-device tracking report is concerned with.

How is ad tracking different from Apple's App Tracking Transparency?

App Tracking Transparency, or ATT, is Apple's specific control over one particular form of tracking on iPhones, not a general anti-tracking setting. Apple's own documentation defines tracking, for ATT purposes, as "linking data collected from an app with data collected from other companies' apps, websites or offline properties, for the purpose of targeted advertising or advertising measurement, or sharing that data with a data broker," as reported by The Mac Observer's review of Apple's documentation. That's a narrow, technical definition — an app can still collect plenty of data about how you use it without that counting as "tracking" under Apple's rule.

When a user declines the ATT prompt, the practical effect is specific: the app can't access the device's advertising identifier, known as the IDFA, and it also can't substitute something else, like an email address, to achieve the same tracking purpose, according to the Mac Observer's reporting on Apple's documentation. That permission isn't locked in forever — every app that has asked for tracking permission is listed under Settings, then Privacy and Security, then Tracking, where it "lists every app that has requested permission and lets a user grant or withdraw it at any time," per the same reporting.

It's also worth separating ATT from Apple's own advertising. ATT governs tracking by third-party apps; Apple documents a distinct setting called Personalized Ads, under Settings, then Privacy and Security, then Apple Advertising, that controls Apple's own ads in the App Store, Apple News and Stocks — a "different, separate control," as the Mac Observer's review of Apple's documentation puts it. For the full walkthrough of adjusting these settings on a phone, see the guide on how to turn off ad tracking on iPhone and Android.

How is ad tracking different from site analytics?

The concern the FTC raises about cross-device tracking isn't that a site counts its visitors — that's ordinary analytics. It's that activity tied to one device gets linked to activity on another device belonging to the same person, usually without the person knowing, and with few ways to stop it. The FTC warns that linking devices together this way can produce larger stores of more sensitive data that then need to be protected, according to the FTC's cross-device tracking report. A site logging how many people viewed a page is a different thing from a network stitching together your laptop, phone and smart TV into one advertising profile.

What are my options to limit ad tracking?

You have both device-level and industry-level tools, and they work at different layers.

On a phone, the ATT control covers tracking by third-party apps, and it's reviewable at any time under Settings, then Privacy and Security, then Tracking, per the Mac Observer's summary of Apple's documentation. The site's own step-by-step guide to turning off ad tracking on iPhone and Android walks through doing this on both platforms, and the guide to removing tracking cookies handles the browser side.

At the industry level, the Network Advertising Initiative overhauled its consumer tools. It retired its older cookie-based and email-based opt-out tools — the NAI states it "has sunset its legacy opt-out tools, including its cookie-based tool for opting out of interest-based advertising and the email-based tool for opting out of audience-matched advertising," according to the NAI's announcement. In their place, the NAI now offers detailed instructions for exercising opt-out choices directly through platform-level controls — "across all of the major technology platforms, including popular web browsers, mobile devices, connected TVs, and streaming devices," per the NAI's consumer resources page. The NAI also launched a browser extension meant to send a standardized opt-out signal, the Global Privacy Control, in a way designed to meet the opt-out requirements of more than a dozen U.S. state privacy laws, according to the same NAI announcement.

None of these tools block every form of tracking at once — ATT covers third-party app tracking on Apple devices, cookie controls cover a browser, and the NAI's platform instructions cover whichever device or platform you apply them to. Using more than one closes more gaps than relying on a single setting.