In computer security, logging in (or logging on, signing in, or signing on) is the process by which an individual gains access to a computer system by identifying and authenticating themselves. The user credentials are typically some form of username and a matching password, and these credentials themselves are sometimes referred to as a login. In practice, modern secure systems often require a second factor such as email or SMS confirmation for extra security. Social login allows a user to use existing user credentials from a social networking service to sign in to or create an account on a new website.

When access is no longer needed, the user can log out, log off, sign out or sign off.