How Ad Networks Build a Profile of You Without Your Knowledge

Ad networks rarely know your name at first. What they build is a profile tied to identifiers: a cookie, a device ID, an email address. Over time, that profile collects enough about your interests, habits and situation to decide which ads you see.

Where the data comes from

• Websites: ad code and tracking pixels on pages report what you view, search for and buy.
• Apps: phone apps share activity with ad networks, often tied to your device's advertising ID.
• Your accounts: what you search, watch and like on platforms such as Google and Meta.
• Purchases: stores upload customer lists, usually as scrambled (hashed) email addresses, to match buyers with ad accounts.
• Data brokers: companies that collect and sell information from public records, loyalty programs and other sources.
• Location: from your IP address, and from apps with location access.

Turning data into a profile

• Interest categories: visits to running sites put you in "fitness"; a mortgage calculator suggests "home buyer".
• Life events: moving, getting married, having a baby, starting a business.
• Purchase intent: someone comparing laptops this week is valuable to advertisers right now.
• Look-alikes: people whose behavior resembles existing customers.

Following you across devices

When you sign in on your phone, laptop and TV, the same account ties them together. Without sign-in, companies try to match devices using shared IP addresses and patterns of use.

How profiles are used

Each time an ad loads, details about you and the page are sent to an auction, where advertisers bid based on how well you match what they want. See how retargeting works.

See what they think they know

• Google: My Ad Center shows topics and brands used for your ads, and lets you turn off personalization.
• Meta: Ad preferences in Accounts Center shows your ad topics and activity from other sites.

Limit the profile

1. Block third-party cookies and reject non-essential cookies on banners.
2. On phones, choose "Ask App Not to Track" on iPhone, or delete the advertising ID on Android.
3. Limit location access for apps.
4. Turn off personalized ads in your accounts.
5. Ghost Browser makes random background visits, adding noise so the interest profile built from your browsing is less accurate.

See tracking without cookies and first- and third-party cookies.