AutoRun and the companion feature AutoPlay are components of the Microsoft Windows operating system that dictate what actions the system takes when a drive is mounted.

AutoRun was introduced in Windows 95 to ease application installation for non-technical users and reduce the cost of software support calls. When an appropriately configured CD-ROM is inserted into a CD-ROM drive, Windows detects the arrival and checks the contents for a special file containing a set of instructions. For a commercial application, these instructions normally initiate installation of the software from the CD-ROM. To maximise the likelihood of installation success, AutoRun also acts when the drive is accessed ("double-clicked") in Windows Explorer (or "My Computer").

Until the introduction of Windows XP, the terms AutoRun and AutoPlay were used interchangeably, developers often using the former term and end users the latter. This tendency is reflected in Windows Policy settings named AutoPlay that change Windows Registry entries named AutoRun, and in the autorun.inf file which causes "AutoPlay" to be added to drives’ context menus. The terminology was of little importance until the arrival of Windows XP and its addition of a new feature to assist users in selecting appropriate actions when new media and devices were detected. This new feature was called AutoPlay and a differentiation between the two terms was created.

AutoRun

AutoRun, a feature of Windows Explorer (actually of the shell32 dll) introduced in Windows 95, enables media and devices to launch programs by use of commands listed in a file called autorun.inf, stored in the root directory of the medium.

Primarily used on installation CD-ROMs, the applications called are usually application installers. The autorun.inf file can also specify an icon which will represent the device visually in Explorer along with other advanced features.

The terms AutoRun and AutoPlay tend to be interchangeably used when referring to the initiating action, the action that detects and starts reading from discovered volumes. The flowchart illustration in the AutoPlay article shows how AutoRun is positioned as a layer between AutoPlay and the Shell Hardware Detection service and may help in understanding the terminology. However, to avoid confusion, this article uses the term AutoRun when referring to the initiating action.

AutoPlay in Windows Vista

AutoPlay is a feature introduced in Windows XP which examines removable media and devices and, based on content such as pictures, music or video files, launches an appropriate application to play or display the content. If available, settings in an autorun.inf file can add to the options presented to the user.

AutoPlay is based on a set of handler applications registered with the AutoPlay system. Each media type (Pictures, Music, Video) can have a set of registered handlers which can deal with playing or display that type of media.

Each hardware device can have a default action occurring on discovery of a particular media type, or the AutoPlay dialog can prompt the user what action to take.