How Amplitude identifies your users

Amplitude identifies your users through three signals: the device ID that each device sends, the Amplitude ID that Amplitude computes to stitch those devices into one profile, and the user ID your product assigns after someone signs in. The device ID arrives automatically from the SDK; Amplitude generates the Amplitude ID once it has enough information to conclusively identify a unique user; and you decide when to set the user ID.

The user ID is a unique identifier for an individual user. Setting one is optional but recommended, because it lets Amplitude reconcile events across every device the same person uses. Set the 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.

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.