Glossary/Privacy and identity/Universal ID (UID2)
What is a universal ID (unified ID) in advertising?
How universal and unified IDs work, the main ID solutions in the market, and what publishers should weigh before passing an ID in their bid requests.
Written by Samhub with the help of AI·Updated 13 Oct 2026·3 min readA universal ID (also called unified ID) is a shared identifier that lets advertisers and publishers recognize the same reader across sites without third-party cookies. Most are based on a hashed email address or a similar signal from a logged-in reader.
Why universal IDs exist
Third-party cookies used to tie a reader’s visits together across sites. As browsers block or limit them, buyers lose that link. Universal IDs offer another way to recognize the same reader, based on information the reader has shared with a site, such as an email at login.
How a universal ID works
A reader logs in
The publisher has an email address or phone number, with consent.
The identifier is hashed
The email is turned into a fixed code that cannot be read back.
The ID provider creates a token
The hashed value is converted into the provider’s ID, often encrypted and refreshed over time.
The ID travels in the bid
Buyers who work with the same ID can recognize the reader and bid accordingly.
Well-known ID solutions
| ID | Based on | Notes |
|---|---|---|
| UID2 | Hashed email or phone | Open source, started by The Trade Desk |
| EUID | Hashed email or phone | European version of UID2 |
| RampID | Email and other signals | From LiveRamp |
| ID5 | Several signals | Independent ID widely used in Europe |
16 pages for publishers, with a checklist for data, programmatic and direct sales.
What publishers should consider
Universal IDs need logged-in readers and clear consent. They can raise addressability, but they also let buyers recognize your readers elsewhere. Many publishers combine IDs with their own first-party audiences, so the value of knowing the reader stays with them.
Addressability at Stampen Media after moving its reader data into one layer with Samhub.
Frequently asked questions
Unified ID 2.0, an open-source identifier started by The Trade Desk. It is created from a hashed and salted email address or phone number.
Yes. Under GDPR a pseudonymous ID that can single out a person is personal data, so consent and transparency rules apply.
Partly. They work for logged-in readers, which is usually a minority of traffic. First-party audiences and contextual data cover the rest.
Many do, because different buyers use different IDs. Each one adds work, so start with the IDs your main buyers ask for.
Related terms
An email address turned into a fixed code that cannot be read back.
Cookies set by another domain than the site you visit.
Readers who are signed in to your site or app.
The share of your audience you can recognize and target.
One EU-based platform to collect, unify and enrich your first-party data, installed with one script.
- A walkthrough with your own data
- Built for publishers and media houses
- 30 minutes, no commitment
Turn your audience data into more revenue
Get the Media Monetization Playbook: how media houses turn first-party data into higher CPMs, data deals and direct sales. With checklists for every step.
