What is cookie syncing, and how does it work?

Cookie syncing, also called cookie matching, connects the different identifiers that advertising platforms assign to the same browser. It allows participating systems to recognise that their separate records relate to a matching browser, without necessarily knowing the person’s name.

A simplified matching process works like this:

• One platform recognises the browser using its own cookie identifier.

• A matching request, sometimes involving tracking pixels or redirects, passes an identifier to another platform.

• The receiving platform associates that identifier with its own record.

This creates a mapping between identifiers; it does not give one company unrestricted access to another company’s cookies. Google’s cookie matching documentation explains an implementation used in advertising.

Matching can support cross-site tracking, audience targeting and advertising measurement. Its effectiveness depends on browser restrictions, available identifiers and user choices.

Where cookie syncing uses storage or access technologies for advertising, UK PECR requires consent. A website’s cookie consent implementation should therefore control the matching requests themselves, including those triggered indirectly by advertising tags.