The mechanism is a list. A tag on the site records a visitor into an audience when they do something, view a page, add to a basket, reach a step and leave, and the ad platform matches that audience when the same person later appears where it sells ads. Google's help describes the practice as using your own data to re-engage people who interacted with your brand before, and lists the shapes it takes: site visitors shown ads on the display network, visitors shown different search ads when they search again, dynamic ads that show the exact products viewed, YouTube viewers, app users, people who filled in a lead form, and Customer Match, where the business uploads a customer list and the platform finds those people in their own accounts.
| List | Who is in it | Used for |
|---|---|---|
| Site visitors | Everyone who loaded a page | Broad awareness; the weakest list |
| Product viewers | People who looked at specific items | Dynamic ads showing those items |
| Basket abandoners | Added to basket, did not pay | The highest-intent list; short window, modest cap |
| Past customers | Uploaded or tagged buyers | Repeat purchase; also the exclusion list for everything else |
| Search behaviour | Past visitors who search again | A higher bid or a different ad on the second search |
Recognising a person again across sites is the part that has changed. The original mechanism was the third-party cookie, set by the ad platform on every site that carried its tag, and Safari blocked those by default in version 13.1 and iOS 13.4 in March 2020, with Firefox taking the same path. Retargeting now rests on a mix: first-party data the business collects and uploads (Customer Match and its equivalents), logged-in identity inside the platforms' own properties, and platform-level matching that a business neither sees nor controls. The practical effect is that the lists built from an email address and a purchase history have become the reliable ones, and the lists built from an anonymous visit have shrunk.
The three settings that decide whether it is welcome
A membership window matched to the buying cycle (days for a pizza, weeks for a sofa). A frequency cap per person per day. An exclusion list of people who already converted, refreshed automatically. Retargeting without the third shows a customer the ad for the item they have just bought.
Attribution flatters the channel. Someone who viewed a product and was going to return anyway is counted as a retargeting conversion when they do, which is why the honest test is a holdout: withhold the ads from a random slice of the list and compare. Most businesses that run the test keep the channel at a smaller budget than before. The Google Ads Display certification covers the list types and the campaign setup; the paid search and display topic covers the whole account, and the marketing automation tools cover the parallel channel of email to known contacts, which is where the same people are reached with consent rather than by recognition.