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",^[1] and these credentials themselves are sometimes referred to as a login, (or a logon or a sign-in or a sign-on).^[2][1] In practice, modern secure systems also often require a second factor for extra security.

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

Logging in is usually used to enter a specific page, which trespassers cannot see.

Once the user is logged in, the login token may be used to track what actions the user has taken while connected to the site.

Logging out may be performed explicitly by the user taking some actions, such as entering the appropriate command, or clicking a website link labeled as such. It can also be done implicitly, such as by the user powering off his or her workstation, closing a web browser window, leaving a website, or not refreshing a webpage within a defined period.

In the case of websites that use cookies to track sessions, when the user logs out, session-only cookies from that site will usually be deleted from the user's computer. In addition, the server invalidates any associations with the session, making any session-handle in the user's cookie store useless. This feature comes in handy if the user is using a public computer or a computer that is using a public wireless connection. As a security precaution, one should not rely on implicit means of logging out of a system, especially not on a public computer, instead one should explicitly log out and wait for the confirmation that this request has taken place.

Logging out of a computer when leaving it is a common security practice, preventing unauthorized users from tampering with it.

There are also people who choose to have a password-protected screensaver set to activate after some period of inactivity, requiring the user to re-enter his or her login credentials to unlock the screensaver and gain access to the system. There can be different methods of logging in that may be via image, fingerprints, eye scan, password (oral or textual input), etc.

The terms became common with the time sharing systems of the 1960s and Bulletin Board Systems (BBS) in the 1970s. Early home computers and personal computers did not generally require them until Windows NT, OS/2 and Linux in the 1990s.

The noun login comes from the verb * (to) log in*, and by analogy with the verb to clock in. Computer systems keep a log of users' access to the system. The term "log" comes from the chip log historically used to record distance travelled at sea, and recorded in a ship's log or log book. To sign in connotes the same idea, but based on the analogy of manually signing a log or visitors book.

While there is no agreed difference in meaning between the three terms (login, logon and sign in), different technical communities tend to prefer one or another – Unix, Novell, Linux and Apple typically using login, with Apple's style guide saying "Users log in to a file server (not log on to)...",^[3] By contrast, Microsoft's style guides traditionally suggested the opposite, and prescribed log on and logon.^[4] In the past Microsoft reserved sign in to when accessing the Internet^[4] but from Windows 8 onwards, have moved to the sign in terminology for local authentication.^[5]