Retargeting is the practice of serving paid ads to people who have already interacted with a brand’s owned surfaces but did not complete the conversion the brand wants. Those interactions are the ordinary signals an ecommerce site produces — a storefront visit, a product page view, an add-to-cart, a started checkout, or an email signup. Google calls the same pattern “remarketing”; the two terms are synonymous in operator usage. Retargeting is the inverse of prospecting, which serves ads to cold users who have not yet engaged with the brand.
How the audience is built and delivered
The audience is built from first-party signals the brand already owns: pixel events fired on the site, ESP segments, CRM uploads, and the customer list maintained inside the storefront (Shopify, for example). From those signals, operators typically maintain a standard set of audiences — all site visitors in a trailing window (commonly 30, 60, or 90 days), product-page viewers, add-to-cart abandoners, checkout abandoners, and past purchasers segmented for upsell or win-back. The trailing windows are convention, not a platform default, and brands tune them to their own buying cycle.
Delivery happens through the ad platform’s native audience builders — Meta Custom Audiences, Google Ads remarketing lists, TikTok Custom Audiences — which read those lists and serve ads to matching users. ESPs like Klaviyo can sync segments directly into Meta Custom Audiences, sparing the team a manual CSV export. The hygiene step that’s easy to skip is the exclusion: drop recent purchasers out of acquisition retargeting so spend doesn’t chase customers who just converted.
Why post-2021 privacy changes matter
The practical effect of recent privacy changes is that browser-side pixel audiences are smaller than they were pre-2021. Two mechanisms drive most of that: Apple’s App Tracking Transparency prompt, which gates the device-level identifier behind a user opt-in that most decline, and Safari’s tracking-cookie restrictions, which shorten the lifetime of the web cookies the browser-side pixel uses to recognize return visitors. The eligible population a pixel can re-find shrank, which is why server-side delivery via the Conversions API and audiences built from first-party data uploads now carry more of the weight than the pixel alone.
The measurement trap
Retargeting ROAS almost always looks great — and that’s misleading. The audience was already high-intent before the ad served, so a meaningful share would have converted from email, organic search, or a direct return visit without the ad ever running. The gap between platform attribution and true contribution is widest on retargeting, precisely because intent is upstream of the ad. The right question is incremental retargeting ROAS, which an incrementality test — typically a holdout — answers. The operator handoff: pair retargeting with prospecting (lookalike audiences built from the retargeting seed are the natural counterpart), keep exclusions clean, and measure incrementality before scaling spend.