> ## Documentation Index
> Fetch the complete documentation index at: https://docs-staging.auth0-mintlify.app/llms.txt
> Use this file to discover all available pages before exploring further.

# Identify Users

> Learn how to uniquely identify users.

There are several options to uniquely identify your users:

1. By the `user_id` property. This is guaranteed to be unique (within a tenant) per user (such as `{identity provider id}|{unique id in the provider}` or `facebook|1234567890`). A user may have the same `user_id` property across multiple Auth0 tenants, but consistency is not guaranteed.

   For [regular](/docs/authenticate/database-connections/auth0-user-store) and [custom](/docs/authenticate/database-connections/custom-db) database connections, avoid creating duplicate `user_id`s within a tenant for different database connections. If you expect possible collisions between IDs from different connections, use a prefix identifying the connection.

2. By a natural key, like the `email` property. We recommend that you use an IdP that requires users to verify their emails and then [enable email verification in Auth0](/docs/manage-users/user-accounts/verify-emails).

   Some IdPs, like Azure AD, can't guarantee emails are verified. In this case, you can [set up email verification for Azure AD and ADFS](/docs/authenticate/identity-providers/enterprise-identity-providers/azuread-adfs-email-verification) or [implement external user verification with Auth0 Forms](https://auth0.com/blog/external-user-verification-with-forms/).