A scroll wheel (or mouse wheel) is a hard plastic or rubbery disc (the "wheel") on a computer mouse that is perpendicular to the mouse surface. It is normally located between the left and right mouse buttons.
It is used, as the name suggests, for scrolling. It can often also be used as a third mouse button by pressing on it. Some modern mice can scroll horizontally as well as vertically, using a tilting scroll-wheel (introduced by Microsoft), scroll ball (found on Apple's Mighty Mouse), pointing stick, or a second scroll wheel. Logitech MicroGear mice (like the MX Revolution and the VX Revolution) use a 14 gram scroll wheel with high rotational inertia like a flywheel that can be used to quickly scroll through long pages and lists.
Scroll wheels are prevalent on modern computer mice. To many users, they have become an integral part of the hardware interface. However, non-wheeled mice are still available.
Scroll wheels can also be found on such handheld devices as portable digital audio players, PDAs, or mobiles phones such as early Sony models and BlackBerry devices. On the Apple iPod, the scroll wheel uses touch sensitive technologies from Synaptics (instead of being mechanical).
The scroll wheel was invented at Microsoft in 1993 by Eric Michelman. The first example of a scrolling mouse is the Genius EasyScroll mouse made by Taiwanese company KYE Systems in 1995, but it was popularized by the Microsoft IntelliMouse in 1996 along with support for the mouse wheel in Microsoft Office 97. It is notably one of the first additions to the basic two-button mouse design used for PCs that became a de facto standard. Its popularity increased with the proliferation of the World Wide Web, where efficient mouse-only scrolling is most useful.
In the 21st century, scroll wheels started appearing on keyboards as well, particularly on Logitech and Microsoft models. It is usually located to the left of the caps lock key. The implementation of scroll wheels on laptop computers has generally faded, while touchpads are often made with the edges acting to scroll the page (rather than to move the pointer), partly making up for the lack of a scroll wheel. On laptops with multitouch capability, scrolling is usually achieved by touching and dragging two fingers on the touchpad at the same time. Many Linux distributions offer a third method of scrolling using the touchpad, where the user will first activate scroll-mode by pressing in a corner of the pad, and than dragging in a circle around the center of the pad, letting go of the touchpad will switch back to the default mouse-mode.
In 1985 NTT, Japan and ETH, Switzerland (Ohno, Fukaya & Nievergeld) jointly developed the first scrolling mouse called the "Mighty Mouse". That had the scroll wheel accessible on the side. Somewhere between 1989 - 1993 Dan Venolia of Apple develops a mouse with a thumb-wheel accessible on the side, and is filed in 1992 as U.S. Patent 5,313,230. In 1995, Mouse Systems released ProAgio, the first commercial mouse with a scroll wheel but few people notice. In 1997 Microsoft files the U.S. Patent 5,912,661 for a mouse with a combined button and scroll wheel.
Other applications 
In many applications (e.g. web browsers), holding down the control key while rolling the scroll wheel causes the text size to increase or decrease, or an image in an image-editing or map-viewing program to zoom in or out, if such a feature is available. Also clicking the link with a scroll wheel button makes the link open in a new tab, and clicking on the opened tab closes it.
In many first-person shooter (FPS) games, the scroll wheel is usually set to change weapons. Because of this some gaming mice, such as Logitech G500, implement a locking feature that lets the mouse switch between scrolling in steps (needed for gaming) and continuous scrolling (good for web browsing). The scroll wheel is also sometimes used to control the zoom of scoped weapons and binoculars. FPS games with an emphasis on realism may choose to use the wheel for changing between stances or adjusting the range settings on iron sights. Since the introduction of tilting scroll-wheels, many FPS games use the left-right motion of the scroll wheel to cause the player to lean left and right. A less common use is to use the mouse wheel to adjust the player's speed of movement.
Real time strategy games use the mouse wheel to change the altitude of the camera. Many RTS games also allow control of the camera with the scroll wheel button held down causing the view to either tilt or pan with mouse movement.
- Coding Horror: Meet The Inventor of the Mouse Wheel
- billbuxton.com - Input Devices: An Illustrated Tour, 2011-03-21
- topdesignmag.com - Mouse History In Photos
- funtrivia.com - Quiet As a Mouse - Fun Facts, Questions, Answers, Information
- Joe Kissell (October 7, 2004, updated 2006). "The Evolution of Scrolling: Reinventing the wheel". Interesting Thing of the Day. Retrieved 2010-02-12.