How Amplitude identifies your users

Amplitude uses three methods to identify your users: device IDs, the Amplitude ID, and user IDs. The device ID comes directly from your users' devices. Amplitude creates the Amplitude ID automatically after it has enough information to conclusively identify a unique user. You set up the user ID.

In Amplitude, a user ID is a unique identifier for an individual user. Using user IDs is optional but recommended. Your product should set a user ID after a user creates an account, logs in, or is otherwise identified in your product.

Amplitude can use a user ID to reconcile events across multiple devices under the same user ID. Amplitude also merges a user's event data on the backend, which connects the correct user ID to any anonymous events the user generated before the user ID assignment. For this reason, you can wait to assign user IDs if that makes sense for your product. This is also why you shouldn't set user IDs for anonymous users.

After you set a user ID in Amplitude, you can't change it.

If your product doesn't assign user IDs, skip this section.

Before you continue to the next step, refer to How Amplitude identifies unique users for details.

Best practices for setting user IDs

• Don't set the user ID if there isn't one. For example, if you set the user ID to the string None for multiple users, Amplitude doesn't recognize those users as separate users. Instead, Amplitude assumes all those users are the same user and groups all events for those users together under that None user ID. You can always set the user ID later.
• Don't assign a user ID that might change. User IDs are fixed forever. For example, don't set a user's email address as their user ID, because email addresses change.
• User IDs are case-sensitive. If you set a user ID in a different case, Amplitude tracks two separate profiles for the same user.
• Assigning user IDs server-side can be tricky. If you encounter issues assigning user IDs, contact Amplitude Support.