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What is user authentication?

Authentication is how an app confirms who a user is, usually a login with an email and password, a social login, or a one-time code. It's paired with authorization, which decides what that verified user is allowed to do. Together they protect accounts and control access to data.

Getting it right matters more than almost anything else in an app, because it guards personal data. Best practice as of 2026 includes hashing passwords (never storing them in plain text), offering multi-factor authentication, and often using a trusted provider rather than building it from scratch.

Many teams use an authentication service (like Firebase Auth, Auth0, or Clerk) instead of building their own. These handle the hard, security-critical parts (secure storage, resets, multi-factor) that are easy to get dangerously wrong alone.

Updated July 2026

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