Realm provides :ref:`many authentication providers <authentication-providers>` to log users into your app. Each provider creates a unique user identity. Realm lets you merge multiple credentials into one user identity.
Credentials must be linked prior to logging a user in. Once credentials are used to login a user, you cannot link that credential anymore. Also, you cannot link multiple email/password credentials together.
Consider an application that offers :ref:`anonymous login
<anonymous-authentication>`, which allows users to explore the app without
registering. If a user wants to continue using the application, they can create
a permanent account by using another authentication provider. Realm
creates a new User object. The app can then link the new identity with the
current user.
Note
Depending on how you have configured email/password authentication, there may be additional steps (confirming the email address, for example) before the new account is created and can be linked.