Lesson 4 of 7

Cookies And Identifiers

Tracking relies on storing an identifier so a tool can recognize a browser across requests. That identifier usually lives in a cookie, and where the cookie is set matters enormously.

First-party cookies

Set on your own domain
Used by GA4 to store a client id
Still allowed, but lifespans are capped

Third-party cookies

Set by another domain in an iframe or pixel
Used for cross-site advertising
Blocked by default in most browsers

Browser features like Safari's Intelligent Tracking Prevention shorten how long script-set cookies survive, sometimes to a day or seven. That is why durable, server-set first-party identifiers are increasingly important.

Key takeaway

Third-party cookies are effectively gone, and first-party cookie lifespans are shrinking. Durable measurement leans on first-party data.