A radar gun or is a small doppler radar device that is used to detect the speed of objects, especially trucks and automobiles for the purpose of speed limit enforcement. The radar gun is used in many sporting events such as pitched baseballs, runners or other moving objects in sports. A radar gun does not return information regarding the object's position. It relies on the Doppler effect applied to a radar beam to measure the speed of objects at which it is pointed. Radar guns may be hand-held, vehicle-mounted or static.

Most of today's radar guns operate at X, K, Ka, IR Band (infrared). An alternative technology, LIDAR, uses pulsed laser light. The radar gun was invented by Bryce K. Brown of Decatur Electronics in March 1954[1], and was first used in Chicago, Illinois by Patrolman Leonard Baldy in April 1954.

Radar guns are radio transmitters and receivers. They send out a radio signal, then receive the same signal back as it bounces off the objects. However, the radar frequency is different when it comes back, and from that difference the radar gun can calculate object speed.

A radar beam is similar to a beam of light in that it spreads out as the distance from the signal origin increases. The signal then bounces off objects in the path of the beam and are reflected back to the gun. The gun uses the Doppler effect to determine the speed of the object in the beam's path. Using a comparison of frequency shift between received images instead of the frequency shift between sent and received frequencies creates what is known as moving radar. Unless the radar system has a provision for converting own-vehicle-speed to an appropriate receiver frequency offset then the radar must be stationary to measure speed.

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