Tracking

Third-Party Cookies

In short

Third-party cookies are set by a different domain than the one you visit, such as an ad network, and let that domain track users across many websites.

Also known as: third-party cookie, third party cookie

Third-party cookies are cookies that don't come from the website you're visiting but from a different domain embedded on it – for example through an ad script, an embedded video or a social media button. Because the same third-party domain appears on thousands of websites, it can recognize one user across all of them.

What they were used for

For a long time, third-party cookies were the foundation of online advertising:

  • Retargeting across websites
  • Conversion tracking by ad networks
  • Frequency capping, i.e. limiting how often someone sees the same ad
  • Building interest profiles

Why they are losing importance

Safari (with Intelligent Tracking Prevention) and Firefox block third-party cookies by default. Add ad blockers and consent requirements in the EU via consent management. Chrome still allows them, but users can block them themselves. For advertisers, this means a noticeable share of conversions simply can't be measured via third-party cookies anymore.

Example

A recruiting company runs ads and measures applications only with a script that relies on third-party cookies. Of 100 actual applications, many come from Safari on iPhones. If the ad account reports only 60 conversions, 40 are missing – and the campaign looks much more expensive than it is.

Third-party cookies in lead generation

If you want to measure leads reliably, rely on first-party data and server-side delivery instead of third-party cookies. Instead of letting the ad platform listen in through the browser, your server sends the conversion directly via the Conversion API – enriched with hashed customer data like email and phone number.

LeadMetrics sends events via the Conversion API to Meta, Google Ads, TikTok and LinkedIn. Why that's more reliable than browser-only tracking is explained in Tracking without CAPI pixel chaos.

Common mistakes

  • Treating ad account numbers as complete: Missing cookies mean missing conversions, not missing customers.
  • Only testing in Chrome: Tracking works on your own computer, but Safari users slip through the cracks.
  • Confusing first party with ‘allowed’: In the EU, first-party marketing tracking usually requires consent as well.

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Frequently asked questions

Safari and Firefox already block them by default. Google did not follow through on its long-announced phase-out in Chrome after several delays. Still, don't rely on them: a large share of users already browse without third-party cookies today.

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Related terms

All terms

First-Party Cookies

First-party cookies are set by the domain you are currently visiting. They store things like logins, shopping carts or tracking IDs for that same website.

Server-Side Tracking

Server-side tracking sends tracking data to ad and analytics platforms from a server instead of the browser – more robust and under your control.

Conversion API · CAPI

A Conversion API (CAPI) is an interface that sends conversions directly from server to server to ad platforms like Meta or Google, without the browser.

Retargeting

Retargeting shows ads to people who have already interacted with your brand, for example by visiting your website without filling out a form yet.

Consent Management

Consent management means collecting, storing and managing your website visitors' consent, for example for analytics and marketing cookies.

Client-Side Tracking

Client-side tracking captures user actions with JavaScript directly in the browser and sends them from there to analytics and ad platforms.