In short
Third-party cookies are identifiers set by a domain other than the site being visited, historically used to track users across the web for targeting and attribution. Safari and Firefox block them by default, Chrome has spent years restricting them, and the practical result is that the ad ecosystem has rebuilt measurement and targeting on first-party data instead. Their decline is the reason Conversions APIs, Enhanced Conversions, server-side tracking, and consent frameworks exist: each is a response to signal that third-party cookies used to carry and no longer can. For a marketer, the useful framing is that third-party cookies were borrowed identity, dependent on the browser permitting cross-site tracking, while first-party data is owned identity, collected directly under your own privacy policy. The migration is neither optional nor reversible. This entry is the site’s canonical explainer for why the cookieless shift happened and what replaced the signal it removed.
Why their decline reshaped ad tech
Cross-site tracking was the plumbing under retargeting pools, view-through attribution, and audience syndication. As browsers closed it off, platforms replaced it with advertiser-supplied signal: Conversions APIs, Enhanced Conversions, hashed customer lists, and consent frameworks that travel with every event.
What replaces them
First-party identifiers (email, phone) captured with consent, hashed at source, and delivered server-side. The businesses that made that move kept their measurement; the ones that waited are still reading partial dashboards.
What replaces third-party cookies for retargeting?
An advertiser that relied on third-party-cookie retargeting pools watched reach shrink as browsers blocked them. Rebuilding retargeting on hashed first-party customer lists and server-side site events restored addressable audiences without depending on cross-site cookies.
Reference: MDN Web Docs, third-party cookies