OAuth 2.0 is the industry standard for authorization, enabling secure delegated access. The framework involves four roles: resource owner, client, authorization server, and resource server. Authorization Code grant is the most secure flow for server-side applications. The PKCE (Proof Key for Code Exchange) extension prevents authorization code interception. Implicit grant is deprecated in favor of PKCE for single-page applications. Client Credentials grant is for server-to-server communication without user involvement. Resource Owner Password Credentials grant is only for trusted applications. The authorization workflow starts with the client redirecting the user to the authorization server. The user authenticates and approves requested scopes. The authorization server returns an authorization code. The client exchanges this code for an access token and optionally a refresh token. Access tokens are typically JWTs containing user identity and permissions. Refresh tokens allow obtaining new access tokens without re-authentication. Implement token validation by signature, expiration, and issuer. Use HTTPS exclusively for all OAuth endpoints. Store tokens securely, never in localStorage. Implement token revocation for logout. Consider using OAuth 2.0 compliant providers like Auth0 or Okta.