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Fast user switching is a feature on some modern multi-user operating systems such as Mac OS X, Windows XP, and Windows Vista. It allows users to switch between user accounts on a single PC without quitting applications and logging out. Analogous functionality was first developed on consumer level hardware by the Xenix operating system which supported multiple virtual consoles. Linux, BSD, and most other PC Unixes adopted virtual terminals and further developed the user interfaces, including allowing users to optionally run separate graphical X Window System sessions.

Many free software environments, such as GNOME, support fast user switching with a drop menu, in a way analogous to OS X. For installations with older environments, the functionality must be enabled in the configuration file of the X display manager (for example GDM) then a hot key sequence such as CTRL-ALT-F8 is pressed. A separate login window will now appear and the second user can login (or even the first user again). Alternatively, in the default install, new X sessions can be started at will by using different display parameters to have them run in different virtual terminals (e.g. "startx -- :1" or "X :1 -query localhost"). Again, hot key sequences allow the user switching to take place.

Fast user switching has various security implications and the handling in different operating systems varies, with each possible choice having advantages and disadvantages. One possibility is that only the first user gets ownership of resources. This is the more simple and secure option; the first user is in control of resources; subsequent users cannot use them, but thereby know that the other user has control. A second option is to allow all users access to shared resources; this is more intuitive and easy, but would allow one user to record another user's conversation, for example. In Windows, shared resources such as sound are available to each session. In Red Hat Linux, the default is that the first session connected gets ownership of the "console resources". Red Hat Linux can be relatively easily reconfigured, however, to provide shared resources instead, either through a dedicated group of console users or by altering the way console ownership is changed.

It should be noted that in Windows XP, fast user switching is unavailable if the computer is on a domain network, thus not a consideration if looking to switch users in a hurry. However, it is available for domain computers starting with Windows Vista.

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