The first to use radio waves to detect the presence of distant metallic objects was Christian Hülsmeyer, who in 1904 demonstrated the feasibility of detecting the presence of a ship in dense fog, but not its distance.

In 1934, Émile Girardeau, working with the first French radar systems, stated he was building radar systems conceived according to the principles stated by Tesla

A radar system has a transmitter that emits radio waves that are reflected by the target and detected by a receiver, typically in the same location as the transmitter. Although the radio signal returned is usually very weak, the signal can be amplified. This enables radar to detect objects at ranges where other emissions, such as sound or visible light, would be too weak to detect.

Modern radar systems are highly sophisticated and can produce detailed information about both stationary and moving objects.

Radar is used in many contexts, including meteorological detection of precipitation, measuring ocean surface waves, air traffic control, police detection of speeding traffic, and by the military.