Basics · also called remarketing
Retargeting
Retargeting is showing ads to people who have already interacted with a brand, for example by visiting its website, using its app or adding a product to a cart without buying.
Retargeting (Google calls it remarketing) follows up with people who already showed interest. When you look at a pair of shoes and then see that exact pair in ads on other sites, that is retargeting.
Mechanically, the advertiser's site or app records visitors using a tracking pixel, SDK or server-side events, and builds a list. That list is then matched against people in ad auctions via cookies, mobile IDs, hashed emails or platform logins, and a DSP or social platform bids to show them ads. Dynamic retargeting uses DCO to show the exact products viewed. Criteo built its business on this technique.
Retargeting often shows excellent ROAS and CPA, partly because many of those people were going to buy anyway, so its incrementality can be lower than it looks under last-click attribution. It is also sensitive to privacy rules and signal loss: it depends on recognising the same person across sites, which App Tracking Transparency and Safari's cookie blocking restrict.
Think of it like this
Retargeting is like a shopkeeper who remembers you tried on a jacket and waves at you from across the street the next day.
An example
An online electronics store in Germany retargets 200,000 cart abandoners; 4% return and buy, with an average order of €180.
Related terms
Targeting
Targeting is choosing which people, contexts, places, devices or times should see an ad, so that the budget is spent on the audiences most likely to respond.
Tracking pixel
A tracking pixel is a tiny piece of code, historically an invisible 1x1 image, that loads when a page or ad is viewed and reports that event to a server.
Third-party cookie
A third-party cookie is a small file set in your browser by a domain other than the site you are visiting, typically ad tech, letting it recognise your browser across sites.
DCO (dynamic creative optimization)
DCO (dynamic creative optimization) is technology that assembles ads automatically from templates and data, changing images, text, offers or prices for each viewer or context and learning which versions perform best.
Incrementality
Incrementality is the extra result, such as sales, installs or visits, caused by advertising that would not have happened without it, usually measured by comparing exposed and unexposed groups.